feat(ci): add Docker publish workflow

Adds a GitHub Actions workflow to build and push a multi-architecture Docker image to Docker Hub.

Key features:
- Triggers on push to the 'develop' branch or manual dispatch.
- Builds a multi-arch image for linux/amd64, linux/arm64, and linux/arm/v7.
- Caches Docker layers and frontend build artifacts to accelerate builds.
- Uses DOCKERHUB_USERNAME secret for the image repository.
- The workflow is skipped if required Docker Hub secrets are not defined.

Author: zuptalo

.github/workflows/docker-publish.yml

@@ -7,6 +7,7 @@ on:
 
 jobs:
   build:
+    if: secrets.DOCKERHUB_USERNAME != '' && secrets.DOCKERHUB_TOKEN != ''
     runs-on: ubuntu-latest
     steps:
       -
@@ -47,6 +48,6 @@ jobs:
           file: ./docker/Dockerfile
           platforms: linux/amd64,linux/arm64,linux/arm/v7
           push: true
-          tags: zuptalo/nginx-proxy-manager:develop
+          tags: ${{ secrets.DOCKERHUB_USERNAME }}/nginx-proxy-manager:develop
           cache-from: type=gha
           cache-to: type=gha,mode=max