Add docker-buildx-plugin to forgejo-runner (#147)

## Summary
- Install `docker-buildx-plugin` alongside `docker-ce-cli` in the forgejo-runner image
- Fixes `docker buildx build` failing with "unknown flag: --tag" from #146

## Test plan
- [ ] Merge and release `forgejo-runner-v2.5.1`
- [ ] Update runner configmap/labels if needed to use new image
- [ ] Re-tag `nettest-v0.11.1` (or `v0.12.0`) to verify build-container workflow succeeds

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/147

containers/forgejo-runner/Dockerfile

@@ -35,7 +35,7 @@ RUN install -m 0755 -d /etc/apt/keyrings \
     && 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 skopeo \
+    && apt-get install -y --no-install-recommends docker-ce-cli docker-buildx-plugin skopeo \
     && rm -rf /var/lib/apt/lists/*
 
 # Install uv (Python package runner for towncrier)

docs/changelog.d/fix-docker-buildx-runner.infra.md

@@ -0,0 +1 @@
+Install `docker-buildx-plugin` in forgejo-runner image to support `docker buildx build`.