CI: don't build the container image by default

.forgejo/workflows/build.yaml

@@ -7,6 +7,11 @@ on:
         required: false
         default: false
         type: boolean
+      build:
+        description: "Build container"
+        required: false
+        default: false
+        type: boolean
 
 jobs:
   build:
@@ -34,11 +39,13 @@ jobs:
         run: pytest -s tests/basic.py
 
       - name: Workaround for bug of podman-login
+        if: ${{ inputs.build }}
         run: |
           mkdir -p $HOME/.docker
           echo "{ \"auths\": {} }" > $HOME/.docker/config.json
 
-      - name: Log in to container registry (with another workaround)
+      - name: Log in to the container registry (with another workaround)
+        if: ${{ inputs.build }}
         uses: actions/podman-login@v1
         with:
           registry: ${{ vars.REGISTRY }}
@@ -46,7 +53,8 @@ jobs:
           password: ${{ secrets.REGISTRY_PASSWORD }}
           auth_file_path: /tmp/auth.json
 
-      - name: Build container
+      - name: Build the container image
+        if: ${{ inputs.build }}
         uses: actions/buildah-build@v1
         with:
           image: treetrail-backend
@@ -54,7 +62,9 @@ jobs:
           containerfiles: |
             ./Containerfile.for_runner
 
-      - uses: actions/push-to-registry@v2
+      - name: Push the image to the registry
+        uses: actions/push-to-registry@v2
+        if: ${{ inputs.build }}
         with:
           registry: "docker://${{ vars.REGISTRY }}"
           image: treetrail-backend