dist(docker): add curl package to Labrinth image, some other minor tweaks (#3915)

* dist(docker): add `.dockerignore` as symlink to `.gitignore`

This ensures that no files outside of version control are transferred to
the Docker build context for Labrinth and Daedalus images, which
significantly improves build speed (if a `target` directory is already
present) and build reproducibility.

* chore(dist/docker): simplify out unneeeded statements, move `SQLX_OFFLINE` env var setting to build command itself

The latter approach ensures that developers building the image locally
don't forget to set `SQLX_OFFLINE`, too.

* dist(docker): add `curl` package to Labrinth image

.github/workflows/labrinth-docker.yml

@@ -18,9 +18,6 @@ on:
 jobs:
   docker:
     runs-on: ubuntu-latest
-    defaults:
-      run:
-        working-directory: ./apps/labrinth
     steps:
       - name: Checkout
         uses: actions/checkout@v2
@@ -38,8 +35,6 @@ jobs:
       - name: Build and push
         id: docker_build
         uses: docker/build-push-action@v2
-        env:
-          SQLX_OFFLINE: true
         with:
           file: ./apps/labrinth/Dockerfile
           push: ${{ github.event_name != 'pull_request' }}