Add custom docker-build image for CI workflows

- Created Dockerfile.build with Docker CLI, Buildx, Git, Bash
- Updated build-ci-image.sh to build both php-ci and docker-build images
- Updated workflow to use docker-build image (no installation needed)
- Updated runner .env to use docker-build:latest instead of docker:latest

.gitea/workflows/production-deploy.yml

@@ -136,12 +136,7 @@ jobs:
 
       - name: Setup Docker Buildx
         run: |
-          # Install Docker Buildx if not present
-          if ! docker buildx version &>/dev/null; then
-            mkdir -p ~/.docker/cli-plugins
-            curl -L "https://github.com/docker/buildx/releases/latest/download/buildx-$(uname -s | tr '[:upper:]' '[:lower:]')-$(uname -m)" -o ~/.docker/cli-plugins/docker-buildx
-            chmod +x ~/.docker/cli-plugins/docker-buildx
-          fi
+          # Buildx ist bereits im docker-build Image installiert
           docker buildx create --name builder --use || docker buildx use builder
           docker buildx inspect --bootstrap