Reclaim runner disk before load: true smoke builds

v1.14.31 publish and validate both hit 'No space left on device' on
single-arch amd64 smoke/validate builds. The image has crossed ~3 GB
and the runner's ~40 GB overlay starts ~70% full, so 'load: true'
peak disk (tarball + unpacked image + buildx cache) no longer fits.

Add a 'Reclaim runner disk' step to validate-base, validate-omos,
smoke-base, smoke-omos. Strips catthehacker-resident toolchains we
never use (hosted-tool-cache, dotnet, android, powershell, swift,
ghc, jvm, microsoft, chromium, boost), then runs 'docker system
prune -af --volumes' + 'docker builder prune -af' against the
runner's dockerd before setup-buildx-action. Expected reclaim is
6-12 GB depending on what's resident.

Deliberately NOT in the per-arch matrix build jobs — push-by-digest
doesn't need it and pruning in parallel jobs risks one job nuking
another's in-flight buildx cache. Also add workflow-level
concurrency on docker-publish.yml so concurrent tag pushes serialize
cleanly.

.gitea/workflows/docker-publish.yml

@@ -5,6 +5,15 @@ on:
     tags:
       - 'v*'
 
+# Serialize concurrent runs of the same workflow on the same ref so the
+# matrix build jobs can't race `docker system prune` in the smoke gates
+# (pruning from one job can nuke another job's in-flight buildx cache).
+# cancel-in-progress: false — tag pushes are release events, we never
+# want to silently drop one.
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: false
+
 # Runner disk pressure notes:
 # Gitea Actions runners use `catthehacker/ubuntu:act-latest` on a shared host
 # with limited overlay space (~40 GB, often 70%+ used at start). Building both
@@ -29,6 +38,34 @@ jobs:
       - name: Force IPv4 for Docker Hub
         run: echo 'precedence ::ffff:0:0/96  100' >> /etc/gai.conf
 
+      # See docker-publish.yml preamble. `load: true` peak disk = tarball
+      # + unpacked image + buildx cache; the image now crosses the 40 GB
+      # runner overlay's starting headroom. Strip catthehacker-resident
+      # toolchains and any stale docker state up front.
+      - name: Reclaim runner disk
+        run: |
+          set -x
+          df -h / || true
+          rm -rf \
+            /opt/hostedtoolcache \
+            /opt/microsoft \
+            /opt/az \
+            /opt/ghc \
+            /usr/local/.ghcup \
+            /usr/share/dotnet \
+            /usr/share/swift \
+            /usr/local/lib/android \
+            /usr/local/share/powershell \
+            /usr/local/share/chromium \
+            /usr/local/share/boost \
+            /usr/lib/jvm 2>/dev/null || true
+          apt-get clean || true
+          rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/* || true
+          docker system df || true
+          docker system prune -af --volumes || true
+          docker builder prune -af || true
+          df -h / || true
+
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v4
         with:
@@ -57,6 +94,30 @@ jobs:
       - name: Force IPv4 for Docker Hub
         run: echo 'precedence ::ffff:0:0/96  100' >> /etc/gai.conf
 
+      - name: Reclaim runner disk
+        run: |
+          set -x
+          df -h / || true
+          rm -rf \
+            /opt/hostedtoolcache \
+            /opt/microsoft \
+            /opt/az \
+            /opt/ghc \
+            /usr/local/.ghcup \
+            /usr/share/dotnet \
+            /usr/share/swift \
+            /usr/local/lib/android \
+            /usr/local/share/powershell \
+            /usr/local/share/chromium \
+            /usr/local/share/boost \
+            /usr/lib/jvm 2>/dev/null || true
+          apt-get clean || true
+          rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/* || true
+          docker system df || true
+          docker system prune -af --volumes || true
+          docker builder prune -af || true
+          df -h / || true
+
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v4
         with: