Explicitely point to docker.io images

This is useful for tools like podman which do not take it for granted
that base images come from hub.docker.com.

Dockerfile

@@ -10,7 +10,7 @@
 # Because this image is still experimental, and we are not quite sure if it's going to 
 # be very useful, we do not provide any usage documentation.
 
-FROM python:3.7
+FROM docker.io/python:3.7
 
 # As per https://github.com/docker/compose/issues/3918
 COPY --from=library/docker:19.03 /usr/local/bin/docker /usr/bin/docker

tutor/templates/build/android/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:18.04
+FROM docker.io/ubuntu:18.04
 MAINTAINER Overhang.io <contact@overhang.io>
 
 RUN apt update && \

tutor/templates/build/forum/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:18.04
+FROM docker.io/ubuntu:18.04
 MAINTAINER Overhang.io <contact@overhang.io>
 
 RUN mkdir /openedx

tutor/templates/build/openedx/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:16.04
+FROM docker.io/ubuntu:16.04
 MAINTAINER Overhang.io <contact@overhang.io>
 
 ############ common to lms & cms