ci: add the NIXPKGS_CHANNEL shortcut

.travis.sh

@@ -7,12 +7,14 @@ set -euo pipefail
 ./build
 
 # default to Docker Hub
+# the user has to set REGISTRY_USER and REGISTRY_PASSWORD
 : "${REGISTRY:=docker.io}"
 : "${IMAGE_PREFIX:=nixpkgs}"
 
-# IMAGE_TAG is provided by .travis.yml
-
-# the user has to set REGISTRY_USER and REGISTRY_PASSWORD
+# either pass those two or set the NIXPKGS_CHANNEL
+: "${IMAGE_TAG:=$NIXPKGS_CHANNEL}"
+: "${NIX_PATH:=nixpkgs=channel:$NIXPKGS_CHANNEL}"
+export NIX_PATH
 
 if [[ "$TRAVIS_BRANCH" = master && -z "$TRAVIS_PULL_REQUEST_BRANCH" ]]; then
   ./docker-login "$REGISTRY_USER" "$REGISTRY_PASSWORD" "$REGISTRY"