Add workflow to manually update image

.github/workflows/update-image.yml

@@ -0,0 +1,46 @@
+# Manually trigger image build and push.
+name: Update Image
+on:
+  workflow_dispatch:
+    inputs:
+      tag:
+        description: Tag for the new image
+        required: true
+jobs:
+  build-and-push-image:
+    if: ${{ github.repository == 'codewars/ocaml' }}
+    name: Build Images
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Get image tag
+      run: |
+        # Remove `v` prefix if given
+        tag=${{ github.event.inputs.tag }}
+        echo "IMAGE_TAG=${tag#v}" >> $GITHUB_ENV
+      shell: bash
+
+    - name: Set up Docker Buildx
+      uses: docker/setup-buildx-action@v1
+
+    - name: Login to GitHub Container Registry
+      uses: docker/login-action@v1
+      with:
+        registry: ghcr.io
+        username: ${{ github.repository_owner }}
+        password: ${{ secrets.GITHUB_TOKEN }}
+
+    - name: Build and push image
+      uses: docker/build-push-action@v2
+      with:
+        context: .
+        push: true
+        tags: |
+          ghcr.io/${{ github.repository }}:latest
+          ghcr.io/${{ github.repository }}:${{ env.IMAGE_TAG }}
+        cache-from: type=gha
+        cache-to: type=gha,mode=max