fix: replace $GITHUB_OUTPUT with inline env vars in CI workflow

Collapse Set image tag, Build Docker image, and Push image steps into
a single step that computes TAG inline, eliminating the dependency on
$GITHUB_OUTPUT which is not reliably available in Gitea Actions runners.
Also moves registry login before the build+push step for correct ordering.

Closes leeworks-agents/gitea-mobile#25

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/build.yaml

@@ -24,25 +24,18 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: Set image tag
-        id: tag
-        run: |
-          TIMESTAMP=$(date +%Y%m%d%H%M%S)
-          SHA=$(echo ${{ gitea.sha }} | cut -c1-7)
-          echo "tag=${TIMESTAMP}-${SHA}" >> $GITHUB_OUTPUT
-
-      - name: Build Docker image
-        run: |
-          docker build -t gitea.leeworks.dev/0xwheatyz/gitea-mobile:${{ steps.tag.outputs.tag }} .
-          docker tag gitea.leeworks.dev/0xwheatyz/gitea-mobile:${{ steps.tag.outputs.tag }} \
-            gitea.leeworks.dev/0xwheatyz/gitea-mobile:latest
-
       - name: Login to Gitea registry
         run: |
           echo "${{ secrets.REGISTRY_PASSWORD }}" | docker login gitea.leeworks.dev \
             -u "${{ secrets.REGISTRY_USERNAME }}" --password-stdin
 
-      - name: Push image
+      - name: Build and push Docker image
         run: |
-          docker push gitea.leeworks.dev/0xwheatyz/gitea-mobile:${{ steps.tag.outputs.tag }}
+          TIMESTAMP=$(date +%Y%m%d%H%M%S)
+          SHA=$(echo ${{ gitea.sha }} | cut -c1-7)
+          TAG="${TIMESTAMP}-${SHA}"
+          docker build -t gitea.leeworks.dev/0xwheatyz/gitea-mobile:${TAG} .
+          docker tag gitea.leeworks.dev/0xwheatyz/gitea-mobile:${TAG} \
+            gitea.leeworks.dev/0xwheatyz/gitea-mobile:latest
+          docker push gitea.leeworks.dev/0xwheatyz/gitea-mobile:${TAG}
           docker push gitea.leeworks.dev/0xwheatyz/gitea-mobile:latest