feat(docker): introduce Java 26 dev/build container and build-secret challenge

[sandeshkuldharme1]

This PR introduces a standardized JDK 22 development/build container bundling wrongsecrets-binaries, and adds a new challenge demonstrating secret leakage in build container layers and intermediate stages.

Motivation

• Environment Parity: Standardizes the build environment using JDK 22 and wrongsecrets-binaries, preventing local JDK version mismatches across contributor machines.
• Security Education: Illustrates risks of passing sensitive credentials via ARG or intermediate build layers in multi-stage Docker builds.

Proposed Changes

• Dev Container: Added .devcontainer/devcontainer.json configured with JDK 22 and docker-in-docker.
• New Challenge: Created ChallengeBuildContainer.java under Docker challenges.
• Unit Tests: Added unit tests in ChallengeBuildContainerTest.java.
• Educational Content: Added challenge_build_container.html with remediation guidance.

Testing Done

• Unit tests passed with ./mvnw clean test
• Container image built and verified locally

---

Checklist

• My code follows the project's style guidelines.
• I have performed a self-review of my code.
• I have added unit tests.

[commjoen]

Can we make this java26?

[sandeshkuldharme1]

Updated to Java 26 (eclipse-temurin:26-jdk-jammy)! Thanks for the review.

[commjoen]

Can you add node js26 as well please?

[commjoen]

Hi @sandeshkuldharme1 ! I think we are missing a few components voor a devcontainer yet. Can you add those please?

[sandeshkuldharme1]

Hi @commjoen, I've updated the devcontainer to Java 26 (eclipse-temurin:26-jdk-jammy), added the missing components (forwarded ports, docker-in-docker, Maven postCreate setup), and fixed the UTF-8 BOM encoding issue. Thanks!

[commjoen]

just to make sure: how can we now use the devcontainer to build the app completely :) ?

[sandeshkuldharme1]

Hi @commjoen,

Here is how you can use the Dev Container to build and run the entire application completely:

1. Open in Dev Container

• In VS Code: Press Ctrl+Shift+P (or Cmd+Shift+P on macOS) $\rightarrow$ select "Dev Containers: Reopen in Container" (or click the green prompt in the bottom-left corner).
• In GitHub Codespaces: Select Code $\rightarrow$ Codespaces $\rightarrow$ Create codespace on this branch.

(The container will automatically set up Java, Docker-in-Docker, forwarded ports, and run the Maven setup).

2. Build the Complete App

Inside the integrated Dev Container terminal, run:

./mvnw clean install

[commjoen]

Hi @commjoen,

Here is how you can use the Dev Container to build and run the entire application completely:

1. Open in Dev Container

• In VS Code: Press Ctrl+Shift+P (or Cmd+Shift+P on macOS)
  →
  select "Dev Containers: Reopen in Container" (or click the green prompt in the bottom-left corner).
• In GitHub Codespaces: Select Code
  →
  Codespaces
  →
  Create codespace on this branch.

(The container will automatically set up Java, Docker-in-Docker, forwarded ports, and run the Maven setup).

2. Build the Complete App

Inside the integrated Dev Container terminal, run:

./mvnw clean install

Can you add this to the contributing.md please?

[commjoen]

It looks like a few more components are missing for a challenge. Can you please have a look at the other challenges to make it complete?