Fix Docker build tag issue in GitHub Actions workflow

The previous tag generation configuration was producing invalid Docker tags
that start with a dash, causing build failures. Changed from using
{{branch}}- prefix to sha- prefix to ensure all tags are properly formatted
and valid for Docker.

Fixes the error: ERROR: failed to build: invalid tag "ghcr.io/pathology-data-mining/mosaic:-6f773e0": invalid reference format

.github/workflows/docker.yml

@@ -61,7 +61,7 @@ jobs:
           type=ref,event=pr
           type=semver,pattern={{version}}
           type=semver,pattern={{major}}.{{minor}}
-          type=sha,prefix={{branch}}-
+          type=sha,prefix=sha-
 
     - name: Extract metadata for Docker Hub
       id: meta-dockerhub