update DockerHub info

create a script that automatically updates the DockerHub project pages

.travis.sh

@@ -6,9 +6,20 @@ set -euo pipefail
 
 ./build
 
+# default to Docker Hub
+: "${REGISTRY:=docker.io}"
+: "${IMAGE_PREFIX:=nixpkgs}"
+
+# IMAGE_TAG is provided by .travis.yml
+
+# the user has to set REGISTRY_USER and REGISTRY_PASSWORD
+
 if [[ "$TRAVIS_BRANCH" = master && -z "$TRAVIS_PULL_REQUEST_BRANCH" ]]; then
-  ./docker-login "$CI_REGISTRY" "$CI_REGISTRY_USER" "$CI_REGISTRY_PASSWORD"
-  ./push-all "$CI_REGISTRY_PREFIX" "$IMAGE_TAG"
+  ./docker-login "$REGISTRY_USER" "$REGISTRY_PASSWORD" "$REGISTRY"
+  ./push-all "$REGISTRY" "$IMAGE_PREFIX" "$IMAGE_TAG"
+  if [[ $REGISTRY = *docker.io ]]; then
+    ./update-dockerhub "$REGISTRY_USER" "$REGISTRY_PASSWORD" "$IMAGE_PREFIX"
+  fi
 else
   echo "=== not pushing on non-master ==="
 fi