Fix Forgejo Actions workflow - use github.* compat context

.forgejo/workflows/build.yaml

@@ -15,8 +15,8 @@ jobs:
         uses: docker/login-action@v3
         with:
           registry: git.mcintire.me
-          username: ${{ gitea.repository_owner }}
-          password: ${{ secrets.GITEA_TOKEN }}
+          username: ${{ github.repository_owner }}
+          password: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
@@ -25,7 +25,7 @@ jobs:
         id: tags
         run: |
           TIMESTAMP=$(date +%s)
-          SHORT_SHA=$(echo "${{ gitea.sha }}" | cut -c1-7)
+          SHORT_SHA=$(echo "${{ github.sha }}" | cut -c1-7)
           TAG="main-${TIMESTAMP}-${SHORT_SHA}"
           echo "tag=${TAG}" >> "$GITHUB_OUTPUT"
           echo "Building image with tag: ${TAG}"