Never fail a container build because the cache was unreachable

Every layer of both architectures built. The job then died exporting to
act_runner's emulated GitHub Actions cache service, which it could not route
to: `GetCacheEntryDownloadURL ... dial tcp 192.168.1.126:40649: no route to
host`.

On a pull_request `push:` is false, so this job pushes nothing and the cache
is its only output — which means a network problem between the buildx
`docker-container` builder and the runner host threw away a complete,
successful validation of the Dockerfile on linux/amd64 and linux/arm64. A
cache is an optimisation; it must degrade to "slow", never to "red".

Only the exporter needs the flag. The import is already non-fatal — the build
ran all 37 layers after warning it could not read the cache.

This does not fix the routing itself, so builds stay uncached until that is
sorted.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0145mQi9NZiCDrznBUEEDE4n

.gitea/workflows/build.yml

@@ -55,5 +55,21 @@ jobs:
             ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:${{ gitea.sha }}
             ghcr.io/shadowdao/triple-c-sandbox:latest
             ghcr.io/shadowdao/triple-c-sandbox:${{ gitea.sha }}
+          # `ignore-error` is what stops a cache failure failing a build that
+          # already succeeded. act_runner emulates the GitHub Actions cache
+          # service on the runner host's LAN address, and the `docker-container`
+          # builder `setup-buildx-action` creates could not route to it —
+          # every layer of both arches built, then the job died on
+          # `GetCacheEntryDownloadURL: no route to host` while exporting.
+          #
+          # On a pull_request `push:` above is false, so this job pushes
+          # nothing and the cache is its only output: failing it discarded a
+          # complete, successful validation of the Dockerfile for both
+          # architectures. A cache is an optimisation and must degrade to
+          # "slow", never to "red".
+          #
+          # The import is already non-fatal — the build ran all 37 layers after
+          # warning that it could not read the cache — so only the exporter
+          # needs the flag.
           cache-from: type=gha
-          cache-to: type=gha,mode=max
+          cache-to: type=gha,mode=max,ignore-error=true