Fix Docker build failing due to username casing

.github/workflows/publish-docker.yml

@@ -33,9 +33,11 @@ jobs:
           username: ${{ github.repository_owner }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
-      - name: Get Version
-        id: get_version
-        run: echo ::set-output name=VERSION::${GITHUB_REF#refs/tags/}
+      - name: Get Variables
+        id: get-variables
+        run: |
+          echo ::set-output name=version::${GITHUB_REF#refs/tags/}
+          echo ::set-output name=gh-username-lower::$(echo ${{ github.repository_owner }} | tr '[:upper:]' '[:lower:]')
 
       - name: Build and push Image
         uses: docker/build-push-action@v2
@@ -43,9 +45,9 @@ jobs:
           platforms: linux/amd64
           tags: |
             ${{ secrets.DOCKERHUB_USERNAME }}/libretranslate:${{ env.TAG }},
-            ghcr.io/${{ github.repository_owner }}/libretranslate:${{ env.TAG }}
+            ghcr.io/${{ steps.get-variables.outputs.gh-username-lower }}/libretranslate:${{ env.TAG }}
           push: true
           cache-from: type=gha
           cache-to: type=gha,mode=max
         env:
-          TAG: ${{ startsWith(github.ref, 'refs/tags/') && steps.get_version.outputs.VERSION || 'latest' }}
+          TAG: ${{ startsWith(github.ref, 'refs/tags/') && steps.get-variables.outputs.version || 'latest' }}