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

docker/ci/Dockerfile.build

@@ -0,0 +1,30 @@
+# Dockerfile für Docker Build Jobs in CI/CD
+# Enthält: Docker CLI, Docker Buildx, Git, Bash
+FROM docker:latest
+
+# Installiere zusätzliche Tools
+RUN apk add --no-cache \
+    git \
+    bash \
+    curl \
+    openssh-client \
+    ca-certificates
+
+# Installiere Docker Buildx
+RUN mkdir -p /root/.docker/cli-plugins && \
+    ARCH=$(uname -m) && \
+    if [ "$ARCH" = "x86_64" ]; then ARCH="amd64"; fi && \
+    curl -L "https://github.com/docker/buildx/releases/download/v0.13.2/buildx-v0.13.2.linux-${ARCH}" \
+    -o /root/.docker/cli-plugins/docker-buildx && \
+    chmod +x /root/.docker/cli-plugins/docker-buildx
+
+# Verifiziere Installation (Buildx wird beim ersten Aufruf initialisiert, daher nur Basis-Checks)
+RUN docker --version && \
+    git --version && \
+    bash --version
+
+# Arbeitsverzeichnis
+WORKDIR /workspace
+
+# Standard-User für CI (UID/GID 1000)
+RUN addgroup -g 1000 ci && adduser -u 1000 -G ci -D ci