ci(docker): self-trigger on workflow file changes

The path filter for pull_request didn't include the workflow file
itself, so PRs that only touch .github/workflows/docker.yml (like the
previous commit on this branch) don't exercise the Docker validate
job. Add the workflow file to its own filter so changes self-validate.

Author: brunobuddy

.github/workflows/docker.yml

@@ -10,6 +10,7 @@ on:
   pull_request:
     branches: [main]
     paths:
+      - ".github/workflows/docker.yml"
       - "docker/Dockerfile"
       - ".dockerignore"
       - "docker/docker-compose.yml"