fix(docker): inject version/commit/date via build args (closes #422)

The Docker build excluded .git via .dockerignore, so Go's debug.ReadBuildInfo()
found no vcs.revision / vcs.time settings and the binaries reported
version=dev, commit=unknown, date=unknown in the web UI.

Two fixes:

1. Dockerfile — declare ARG VERSION/COMMIT/DATE (default to dev/unknown/unknown
   so local docker build still works) and pass them to both go build commands
   via -X main.version/commit/date ldflags. Also add the -trimpath and -s -w
   flags that the Makefile's BUILDFLAGS already uses but the Dockerfile was
   missing.

2. release.yml — add a 'Set build date' step, then pass build-args with
   VERSION, COMMIT (full SHA), and DATE to both docker/build-push-action
   steps. The .git exclusion in .dockerignore stays correct; version info
   is now supplied explicitly instead of being read from VCS at build time.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/release.yml

@@ -523,6 +523,10 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
+      - name: Set build date
+        id: build_date
+        run: echo "date=$(date -u +%Y-%m-%d)" >> $GITHUB_OUTPUT
+
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@d7f5e7f509e45cec5c76c4d5afdd7de93d0b3df5 # v4.1.0
 
@@ -552,6 +556,10 @@ jobs:
           push: true
           tags: ${{ steps.meta-service.outputs.tags }}
           labels: ${{ steps.meta-service.outputs.labels }}
+          build-args: |
+            VERSION=v${{ needs.validate.outputs.version }}
+            COMMIT=${{ github.sha }}
+            DATE=${{ steps.build_date.outputs.date }}
           cache-from: type=gha
           cache-to: type=gha,mode=max
 
@@ -574,6 +582,10 @@ jobs:
           push: true
           tags: ${{ steps.meta-web.outputs.tags }}
           labels: ${{ steps.meta-web.outputs.labels }}
+          build-args: |
+            VERSION=v${{ needs.validate.outputs.version }}
+            COMMIT=${{ github.sha }}
+            DATE=${{ steps.build_date.outputs.date }}
           cache-from: type=gha
           cache-to: type=gha,mode=max