fix: use static 'sha-' prefix for Docker SHA tag

{{branch}} is empty when triggered by a tag event, producing an invalid
tag like '-b01bd4fXXX' (leading dash). Replace with static 'sha-' prefix.

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

Author: johnzastrow

.github/workflows/docker-build.yml

@@ -57,7 +57,7 @@ jobs:
             type=semver,pattern=v{{major}}.{{minor}}
             type=semver,pattern=v{{major}}
             type=raw,value=latest,enable={{is_default_branch}}
-            type=sha,prefix={{branch}}-
+            type=sha,prefix=sha-
             type=raw,value=build-${{ github.run_number }}
           labels: |
             org.opencontainers.image.title=ActaLog