Add skopeo to forgejo-runner image

Pre-install skopeo for pushing images to zot registry.
Docker 27's manifest format has compatibility issues with zot,
so we use skopeo for the push step.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

containers/forgejo-runner/Dockerfile

@@ -29,12 +29,13 @@ RUN curl -fsSL https://deb.nodesource.com/setup_20.x | bash - \
     && rm -rf /var/lib/apt/lists/*
 
 # Install Docker CLI (for container builds - daemon accessed via socket mount)
+# and skopeo (for pushing images to zot registry - Docker 27 manifest compat issues)
 RUN install -m 0755 -d /etc/apt/keyrings \
     && curl -fsSL https://download.docker.com/linux/debian/gpg -o /etc/apt/keyrings/docker.asc \
     && chmod a+r /etc/apt/keyrings/docker.asc \
     && echo "deb [arch=$(dpkg --print-architecture) signed-by=/etc/apt/keyrings/docker.asc] https://download.docker.com/linux/debian $(lsb_release -cs) stable" > /etc/apt/sources.list.d/docker.list \
     && apt-get update \
-    && apt-get install -y --no-install-recommends docker-ce-cli \
+    && apt-get install -y --no-install-recommends docker-ce-cli skopeo \
     && rm -rf /var/lib/apt/lists/*
 
 # Default to bash