.docker-hub: make context for varnish smaller

That we don't have to build the image when it did not change.
We don't copy anything into the image, so we use no context.

Author: BacLuc

.docker-hub/docker-compose.yml

@@ -24,5 +24,5 @@ services:
   varnish-image:
     image: ${REGISTRY:-docker.io}/${REPO_OWNER:-ecamp}/ecamp3-varnish:${VERSION:-latest}
     build:
-      context: ../
+      context: .
       dockerfile: .docker-hub/varnish/Dockerfile