Fix Docker tag format in build pipeline

Remove the prefix={{branch}}- from the sha tag type which was causing
invalid tag formats like ":-cbc2c2c" when building pull requests.
The {{branch}} placeholder becomes empty for PRs, leaving only the dash
prefix which creates an invalid Docker tag.

Changed from: type=sha,prefix={{branch}}-
Changed to: type=sha

This generates valid tags like "sha-cbc2c2c" for all events.

.github/workflows/docker-build.yml

@@ -60,7 +60,7 @@ jobs:
             type=semver,pattern={{version}}
             type=semver,pattern={{major}}.{{minor}}
             type=semver,pattern={{major}}
-            type=sha,prefix={{branch}}-
+            type=sha
             type=raw,value=latest,enable={{is_default_branch}}
 
       - name: Build and push Docker image