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

.dockerignore

@@ -0,0 +1 @@
+.gitignore