switch CI to use GitHub Package Registry

Dockerfile

@@ -1,13 +1,16 @@
-# Custom Hugo Extended fork, pre-built on Docker Hub:
-# https://hub.docker.com/r/jakejarvis/hugo-custom
-# Base Dockerfile source: https://go.jarv.is/hugo-dockerfile
+# Custom Hugo Extended fork, pre-built on GitHub Package Registry:
+#     https://github.com/jakejarvis/hugo-custom/packages
+#     https://github.com/jakejarvis/hugo-custom/blob/master/Dockerfile
 # 
 # Usage:
-#      docker build -t jarv.is:develop -f Dockerfile .
-#      docker run -v $(pwd):/src -p 1313:1313 jarv.is:develop
+#     docker build -t jarv.is:develop -f Dockerfile .
+#     docker run -v $(pwd):/src -p 1313:1313 jarv.is:develop
 # ...then open http://localhost:1313 for the live-refresh server.
 
-FROM jakejarvis/hugo-custom:latest
+FROM docker.pkg.github.com/jakejarvis/hugo-custom/hugo-custom:latest
+
+# verify everything's OK
+RUN /usr/bin/hugo version
 
 # add site source as volume
 VOLUME /src