CI: make building container image optional; tag it with the git version

.forgejo/workflows/build.yaml

@@ -1,5 +1,12 @@
 on:
   push:
+  workflow_dispatch:
+    inputs:
+      build:
+        description: "Build container"
+        required: false
+        default: false
+        type: boolean
 
 jobs:
   build:
@@ -24,20 +31,28 @@ jobs:
       - name: Build package (transpile ts => js)
         run: ng build
 
+      - name: Get the version from git
+        id: version
+        if: ${{ inputs.build }}
+        run: echo "version=$(git describe --dirty --tags)" >> $GITHUB_OUTPUT
+
       - name: Build container
+        if: ${{ inputs.build }}
         uses: actions/buildah-build@v1
         with:
           image: treetrail-frontend
-          #tags: foo # v1 ${{ github.sha }}
+          tags: ${{ steps.version.outputs.version }}
           containerfiles: |
             ./Containerfile
 
       - 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)
+        if: ${{ inputs.build }}
         uses: actions/podman-login@v1
         with:
           registry: ${{ vars.REGISTRY }}
@@ -46,7 +61,8 @@ jobs:
           auth_file_path: /tmp/auth.json
 
       - uses: actions/push-to-registry@v2
+        if: ${{ inputs.build }}
         with:
           registry: "docker://${{ vars.REGISTRY }}"
           image: treetrail-frontend
-          tags: latest
+          tags: ${{ steps.version.outputs.version }}