ci: Allow manual execution of Docker workflow

.github/workflows/docker.yml

@@ -13,6 +13,21 @@ on:
   release:
     types: [ published ]
 
+  workflow_dispatch:
+    inputs:
+      release_tag:
+        description: A release version of Recyclarr to build the image for.
+        required: true
+        type: string
+      should_publish:
+        description: If enabled, the image will get published to GHCR.
+        required: false
+        default: false
+        type: boolean
+
+env:
+  SHOULD_PUBLISH: ${{ github.event_name == 'release' || github.event.inputs.should_publish == 'true' }}
+
 jobs:
   docker:
     name: Build & Push Docker Image
@@ -39,7 +54,7 @@ jobs:
             type=semver,pattern={{major}},value=${{ github.event.release.tag_name }}
 
       - name: Login to GHCR
-        if: github.event_name == 'release'
+        if: env.SHOULD_PUBLISH
         uses: docker/login-action@v1
         with:
           registry: ghcr.io
@@ -50,7 +65,7 @@ jobs:
         uses: docker/build-push-action@v2
         with:
           context: ./docker
-          push: ${{ github.event_name == 'release' }}
+          push: ${{ env.SHOULD_PUBLISH }}
           no-cache: true
           build-args: |
             REPOSITORY=${{ github.repository }}