ci(web-server): limit workflow concurrency to avoid GHA rate limits

There seems to be an issue currently where concurrent builds cause the
Docker GHA cache exporter to trip over GitHub throttling limits.

Background reading:

- buildx failed with: error: failed to solve: blob not found #422
docker/build-push-action#422

- Copy from previous stage fails #681
docker/buildx#681

- GHA export cache fails with mode=max #2276
moby/buildkit#2276

Try to work around this by limiting the overall concurrency to 1 for
this workflow.

.github/workflows/web-server.yaml

@@ -4,6 +4,20 @@ defaults:
   run:
     working-directory: web-server
 
+# There seems to be an issue currently where concurrent builds cause the Docker GHA cache exporter
+# to trip over GitHub throttling limits.
+#
+# Background reading:
+#
+# - buildx failed with: error: failed to solve: blob not found #422 https://github.com/docker/build-push-action/issues/422
+# - Copy from previous stage fails #681 https://github.com/docker/buildx/issues/681
+# - GHA export cache fails with mode=max #2276 https://github.com/moby/buildkit/issues/2276
+#
+# Try to work around this by limiting the overall concurrency to 1 for this workflow.
+#
+# Docs: https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#concurrency
+concurrency: workflow-${{ github.workflow }}
+
 jobs:
   build:
     runs-on: ubuntu-latest