add more Dockerfile comments

Dockerfile

@@ -1,3 +1,11 @@
+#
+# This Dockerfile builds a recent curl with HTTP/2 client support, using
+# a recent nghttp2 build.
+#
+# See the Makefile for how to tag it. If Docker and that image is found, the
+# Go tests use this curl binary for integration tests.
+#
+
 FROM ubuntu:trusty
 
 RUN apt-get update && \