[varnish] adjust ENV variable
[varnish] adjust ENV variable

`gcc` is already installed, so it's not needed in `VMOD_DEPS`, plus
trying to uninstall the package will uninstall `varnish` too.

docker-library/official-images,docker-library/official-images,infosiftr/stackbrew,neo-technology/docker-official-images,infosiftr/stackbrew,infosiftr/stackbrew,docker-library/official-images,neo-technology/docker-official-images,infosiftr/stackbrew,neo-technology/docker-official-images,neo-technology/docker-official-images,neo-technology/docker-official-images,neo-technology/docker-official-images,docker-library/official-images,infosiftr/stackbrew,infosiftr/stackbrew,neo-technology/docker-official-images,docker-library/official-images,docker-library/official-images,infosiftr/stackbrew,infosiftr/stackbrew,neo-technology/docker-official-images,neo-technology/docker-official-images,neo-technology/docker-official-images,infosiftr/stackbrew,infosiftr/stackbrew,docker-library/official-images,infosiftr/stackbrew,docker-library/official-images,docker-library/official-images,infosiftr/stackbrew,infosiftr/stackbrew,docker-library/official-images,docker-library/official-images,neo-technology/docker-official-images,docker-library/official-images,docker-library/official-images,infosiftr/stackbrew,neo-technology/docker-official-images,neo-technology/docker-official-images,neo-technology/docker-official-images,docker-library/official-images