Add GitHub Actions workflow for Docker image CI

Author: lygris

.github/workflows/docker-build.yml

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+name: Docker Image CI
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v4
+
+    - name: Build and tag the Docker image
+      run: |
+        docker build . --file Dockerfile --tag doh-server:latest
+        # If you want to push to Docker Hub, uncomment the following lines
+        # and ensure DOCKER_USERNAME and DOCKER_PASSWORD are set as GitHub Secrets.
+        # echo "${{ secrets.DOCKER_PASSWORD }}" | docker login -u "${{ secrets.DOCKER_USERNAME }}" --password-stdin
+        # docker tag doh-server:latest your_dockerhub_username/doh-server:latest
+        # docker push your_dockerhub_username/doh-server:latest

README.md

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+# doh-server
+
+This repository contains a Dockerized DNS-over-HTTPS (DoH) server, built using `dnscrypt/doh-server`. It provides a simple way to run your own DoH resolver.
+
+## Table of Contents
+
+*   [Getting Started](#getting-started)
+*   [Building the Docker Image](#building-the-docker-image)
+*   [Running the Docker Container](#running-the-docker-container)
+*   [Usage](#usage)
+*   [GitHub Actions CI/CD](#github-actions-cicd)
+
+## Getting Started
+
+To run this DoH server, you will need [Docker](https://www.docker.com/get-started/) installed on your system.
+
+## Building the Docker Image
+
+You can build the Docker image locally using the provided `Dockerfile`:
+
+```bash
+docker build -t doh-server:latest .
+```
+
+This command will create a Docker image named `doh-server` with the tag `latest`.
+
+## Running the Docker Container
+
+Once the image is built, you can run the DoH server in a Docker container:
+
+```bash
+docker run -d -p 8053:8053 --name doh-server doh-server:latest
+```
+
+*   `-d`: Runs the container in detached mode (in the background).
+*   `-p 8053:8053`: Maps port `8053` on your host machine to port `8053` inside the container. The `doh-server` listens on port `8053` by default.
+*   `--name doh-server`: Assigns a name to your container for easier management.
+
+## Usage
+
+After the container is running, the DoH server will be accessible on `http://localhost:8053`. You can test it using `curl`:
+
+```bash
+curl -H 'accept: application/dns-json' 'http://localhost:8053/dns-query?dns=q80BAAABAAAAAAAAA3d3dwdleGFtcGxlA2NvbQAAAQAB'
+```
+
+This command queries for `www.example.com` using the DoH server.
+
+## GitHub Actions CI/CD
+
+This repository includes a GitHub Actions workflow (`.github/workflows/docker-build.yml`) that automatically builds the Docker image whenever changes are pushed to the `main` branch. This ensures that your Docker image is always up-to-date with the latest code.