diff --git a/.circleci/config.yml b/.circleci/config.yml
deleted file mode 100644
index 331731d..0000000
--- a/.circleci/config.yml
+++ /dev/null
@@ -1,98 +0,0 @@
-version: 2.1
-
-workflows:
-  version: 2
-
-  main:
-    jobs:
-      - lintSyntax
-      - lintStaticAnalysis
-      - checkBcBreaks
-      - test80
-      - test81
-      - test82
-      - test83
-      - test84
-
-jobs:
-  lintStaticAnalysis:
-    docker:
-      - image: cimg/php:8.1
-    steps:
-      - checkout
-      - run:
-          name: "Install dependencies"
-          command: sudo composer self-update && composer install -n --prefer-dist
-      - run:
-          name: Static analysis
-          command: composer run lint-static-analysis
-
-  lintSyntax:
-    docker:
-      - image: cimg/php:8.1
-    steps:
-      - checkout
-      - run:
-          name: "Install dependencies"
-          command: sudo composer self-update && composer install -n --prefer-dist
-      - run:
-          name: Install php-cs-fixer
-          command: mkdir -p tools/php-cs-fixer && composer require --working-dir=tools/php-cs-fixer friendsofphp/php-cs-fixer
-      - run:
-          name: Run syntax test
-          command: tools/php-cs-fixer/vendor/bin/php-cs-fixer fix --dry-run --stop-on-violation -v
-
-  checkBcBreaks:
-    docker:
-      - image: cimg/php:8.1
-    steps:
-      - checkout
-      - run:
-          name: "Install dependencies"
-          command: sudo composer self-update && composer install -n --prefer-dist
-      - run:
-          name: Check for backwards-compatibility breaks
-          command: composer run test-for-bc-breaks || true
-
-
-  test80:
-    docker:
-      - image: cimg/php:8.0
-    steps:
-      - checkout
-      - run: { name: "Install dependencies", command: "sudo composer self-update && composer install -n --prefer-dist" }
-      - run: { name: "Test suite", command: "composer test-quick-fail" }
-
-  test81:
-    docker:
-      - image: cimg/php:8.1
-    steps:
-      - checkout
-      - run: { name: "Install dependencies", command: "sudo composer self-update && composer install -n --prefer-dist" }
-      - run: { name: "Test suite", command: "composer test-quick-fail" }
-
-  test82:
-    docker:
-      - image: cimg/php:8.2
-    steps:
-      - checkout
-      - run: { name: "Install dependencies", command: "sudo composer self-update && composer install -n --prefer-dist" }
-      # This test is knee-kapped for now, since a PHP bug causes the tests to segfault. See v7.0.0 changelog.
-      - run: { name: "Test suite", command: "composer test-quick-fail" }
-
-  test83:
-    docker:
-      - image: cimg/php:8.3
-    steps:
-      - checkout
-      - run: { name: "Install dependencies", command: "sudo composer self-update && composer install -n --prefer-dist" }
-      - run: { name: "Test suite", command: "composer test-quick-fail" }
-
-  test84:
-    docker:
-      - image: cimg/php:8.4
-    steps:
-      - checkout
-      - run: { name: "Install dependencies", command: "sudo composer self-update && composer install -n --prefer-dist" }
-      - run: { name: "Test suite", command: "composer test-quick-fail" }
-
diff --git a/.devcontainer/Dockerfile b/.devcontainer/Dockerfile
new file mode 100644
index 0000000..f72faf2
--- /dev/null
+++ b/.devcontainer/Dockerfile
@@ -0,0 +1,20 @@
+ARG PHP_VERSION=8.1
+FROM php:${PHP_VERSION}-cli
+
+RUN groupadd --gid 1000 vscode \
+    && useradd --uid 1000 --gid 1000 --create-home --shell /bin/bash vscode \
+    && apt-get update \
+    && export DEBIAN_FRONTEND=noninteractive \
+    && apt-get install -y --no-install-recommends \
+        curl \
+        git \
+        libicu-dev \
+        libonig-dev \
+        libxml2-dev \
+        libzip-dev \
+        unzip \
+        zip \
+    && docker-php-ext-install -j"$(nproc)" bcmath dom intl mbstring soap xml zip \
+    && curl -fsSL https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer \
+    && git config --system --add safe.directory '*' \
+    && rm -rf /var/lib/apt/lists/*
diff --git a/.devcontainer/devcontainer.json b/.devcontainer/devcontainer.json
new file mode 100644
index 0000000..ed8872c
--- /dev/null
+++ b/.devcontainer/devcontainer.json
@@ -0,0 +1,21 @@
+{
+  "name": "contentful-core.php",
+  "build": {
+    "dockerfile": "Dockerfile",
+    "context": "..",
+    "args": {
+      "PHP_VERSION": "${localEnv:PHP_VERSION:8.1}"
+    }
+  },
+  "workspaceFolder": "/workspaces/${localWorkspaceFolderBasename}",
+  "remoteUser": "vscode",
+  "postCreateCommand": ".devcontainer/post-create.sh",
+  "customizations": {
+    "vscode": {
+      "extensions": [
+        "bmewburn.vscode-intelephense-client",
+        "xdebug.php-debug"
+      ]
+    }
+  }
+}
diff --git a/.devcontainer/post-create.sh b/.devcontainer/post-create.sh
new file mode 100755
index 0000000..a63ef26
--- /dev/null
+++ b/.devcontainer/post-create.sh
@@ -0,0 +1,29 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+temp_composer="composer.devcontainer.json"
+temp_lock="composer.devcontainer.lock"
+
+cleanup() {
+  rm -f "$temp_composer" "$temp_lock"
+}
+
+trap cleanup EXIT
+
+# Install a devcontainer-only dependency set without the BC checker. This repo
+# does not commit a lockfile, and roave/backward-compatibility-check can force
+# incompatible resolutions during post-create; the dedicated BC CI job still
+# exercises that tooling separately.
+php <<'PHP'
+<?php
+
+$composer = json_decode(file_get_contents('composer.json'), true, 512, JSON_THROW_ON_ERROR);
+unset($composer['require-dev']['roave/backward-compatibility-check']);
+
+file_put_contents(
+    'composer.devcontainer.json',
+    json_encode($composer, JSON_PRETTY_PRINT | JSON_UNESCAPED_SLASHES).PHP_EOL
+);
+PHP
+
+COMPOSER="$temp_composer" composer install -n --prefer-dist
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..7563012
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,87 @@
+name: CI
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+    branches: [master]
+
+permissions:
+  contents: read
+
+jobs:
+  lint-syntax:
+    name: Lint syntax (PHP 8.1)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v5
+        with:
+          fetch-depth: 0
+
+      - name: Install devcontainer CLI
+        run: npm install -g @devcontainers/cli@0
+
+      - name: Run syntax checks in dev container
+        env:
+          PHP_VERSION: "8.1"
+        run: |
+          devcontainer up --workspace-folder .
+          devcontainer exec --workspace-folder . bash -lc "git ls-files -z '*.php' | xargs -0 -n1 php -l"
+
+  static-analysis:
+    name: Static analysis (PHP 8.1)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v5
+        with:
+          fetch-depth: 0
+
+      - name: Install devcontainer CLI
+        run: npm install -g @devcontainers/cli@0
+
+      - name: Run static analysis in dev container
+        env:
+          PHP_VERSION: "8.1"
+        run: |
+          devcontainer up --workspace-folder .
+          devcontainer exec --workspace-folder . bash -lc "composer run lint-static-analysis"
+
+  backwards-compatibility:
+    name: Backwards compatibility (PHP 8.1)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v5
+        with:
+          fetch-depth: 0
+
+      - name: Install devcontainer CLI
+        run: npm install -g @devcontainers/cli@0
+
+      - name: Check backwards compatibility in dev container
+        env:
+          PHP_VERSION: "8.1"
+        run: |
+          devcontainer up --workspace-folder .
+          devcontainer exec --workspace-folder . bash -lc "composer install -n --prefer-dist && composer run test-for-bc-breaks || true"
+
+  test:
+    name: Test (PHP ${{ matrix.php-version }})
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        php-version: ["8.0", "8.1", "8.2", "8.3", "8.4"]
+    steps:
+      - uses: actions/checkout@v5
+        with:
+          fetch-depth: 0
+
+      - name: Install devcontainer CLI
+        run: npm install -g @devcontainers/cli@0
+
+      - name: Run test suite in dev container
+        env:
+          PHP_VERSION: ${{ matrix.php-version }}
+        run: |
+          devcontainer up --workspace-folder .
+          devcontainer exec --workspace-folder . bash -lc "composer test-quick-fail"
diff --git a/.github/workflows/codeql.yml b/.github/workflows/codeql.yml
deleted file mode 100644
index de95c07..0000000
--- a/.github/workflows/codeql.yml
+++ /dev/null
@@ -1,32 +0,0 @@
----
-name: "CodeQL Scan for GitHub Actions Workflows"
-
-on:
-  push:
-    branches: [master]
-    paths: [".github/workflows/**"]
-  pull_request:
-    branches: [master]
-    paths: [".github/workflows/**"]
-
-jobs:
-  analyze:
-    name: Analyze GitHub Actions workflows
-    runs-on: ubuntu-latest
-    permissions:
-      actions: read
-      contents: read
-      security-events: write
-
-    steps:
-      - uses: actions/checkout@v4
-
-      - name: Initialize CodeQL
-        uses: github/codeql-action/init@v3
-        with:
-          languages: actions
-
-      - name: Run CodeQL Analysis
-        uses: github/codeql-action/analyze@v3
-        with:
-          category: actions
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
new file mode 100644
index 0000000..a28d8f2
--- /dev/null
+++ b/CONTRIBUTING.md
@@ -0,0 +1,39 @@
+# Contributing
+
+Thanks for helping improve `contentful-core.php`.
+
+## Development with Dev Containers
+
+This repository includes a `.devcontainer` configuration for a reproducible local setup. GitHub Actions uses the same devcontainer configuration for CI.
+
+### Visual Studio Code
+
+Open the repository in Visual Studio Code, install the Dev Containers extension if needed, then run `Dev Containers: Reopen in Container`. Wait for the container build and post-create setup to finish.
+
+### Terminal or other editors
+
+Install Docker and the Dev Container CLI (`npm install -g @devcontainers/cli`). From the repository root, run:
+
+```bash
+devcontainer up --workspace-folder .
+devcontainer exec --workspace-folder . bash
+```
+
+### Verify the environment
+
+```bash
+composer test-quick-fail
+```
+
+## Other Useful Commands
+
+```bash
+composer run lint-static-analysis
+composer run test-for-bc-breaks
+```
+
+## Pull Requests
+
+1. Fork the repository and create a branch for your change.
+2. Run the relevant checks from the dev container.
+3. Open a pull request with a short summary of the change and any follow-up context.
diff --git a/README.md b/README.md
index 6f6a3eb..188686f 100644
--- a/README.md
+++ b/README.md
@@ -3,7 +3,7 @@
 [![Packagist](https://img.shields.io/packagist/v/contentful/core.svg?style=for-the-badge)](https://packagist.org/packages/contentful/core)
 [![PHP from Packagist](https://img.shields.io/packagist/php-v/contentful/core.svg?style=for-the-badge)](https://packagist.org/packages/contentful/core)
 [![Packagist](https://img.shields.io/github/license/contentful/contentful-core.php.svg?style=for-the-badge)](https://packagist.org/packages/contentful/core)
-<img height="28px" src="https://circleci.com/gh/contentful/contentful-core.php.svg?style=shield"/>
+[![CI](https://github.com/contentful/contentful-core.php/actions/workflows/ci.yml/badge.svg)](https://github.com/contentful/contentful-core.php/actions/workflows/ci.yml)
 
 ## What is Contentful?
 
@@ -17,6 +17,27 @@ This package requires PHP 8.0 or above.
 
 This package provides foundation components for both [Delivery SDK](https://github.com/contentful/contentful.php) and [Management SDK](https://github.com/contentful/contentful-management.php), and it is not meant for being used from third party packages.
 
+## Development with Dev Containers
+
+For a reproducible local setup, this repository includes a `.devcontainer` configuration and GitHub Actions uses the same setup for CI.
+
+In Visual Studio Code, open the repository and run `Dev Containers: Reopen in Container`.
+
+Without Visual Studio Code, install Docker and the Dev Container CLI (`npm install -g @devcontainers/cli`), then run:
+
+```bash
+devcontainer up --workspace-folder .
+devcontainer exec --workspace-folder . bash
+```
+
+After the container is ready, run:
+
+```bash
+composer test-quick-fail
+```
+
+See [CONTRIBUTING.md](CONTRIBUTING.md) for the full contributor workflow.
+
 ## License
 
 
diff --git a/src/Api/DateTimeImmutable.php b/src/Api/DateTimeImmutable.php
index add8609..b068811 100644
--- a/src/Api/DateTimeImmutable.php
+++ b/src/Api/DateTimeImmutable.php
@@ -28,7 +28,7 @@ public function formatForJson(): string
         $milliseconds = floor($date->format('u') / 1000);
 
         if ($milliseconds > 0) {
-            $result .= '.'.mb_str_pad((string) $milliseconds, 3, '0', \STR_PAD_LEFT);
+            $result .= '.'.str_pad((string) $milliseconds, 3, '0', \STR_PAD_LEFT);
         }
 
         return $result.'Z';
diff --git a/src/File/UrlOptionsInterface.php b/src/File/UrlOptionsInterface.php
index d2ea167..11d2e47 100644
--- a/src/File/UrlOptionsInterface.php
+++ b/src/File/UrlOptionsInterface.php
@@ -3,7 +3,7 @@
 /**
  * This file is part of the contentful/contentful-core package.
  *
- * @copyright 2015-2025 Contentful GmbH
+ * @copyright 2015-2026 Contentful GmbH
  * @license   MIT
  */