Streamline Dockerfile for caching and size

Move package installation to top of file to enable layer caching
in Docker. Use multi stage builds to reduce final image size.
Ignore everything but essential files when creating Docker build
context.

.dockerignore

@@ -1,7 +1,6 @@
-.git
-docs/_build
-node_modules
-test_data
-light
-config.json
-*.mbtiles
+*
+!src
+!public
+!package.json
+!package-lock.json
+!run.sh