Some backports from work on #46 to greatly speed up Circle builds

This backports from topic-docker the base image, which we can now use in
Circle to MASSIVELY speed up our CI situation.

The base image can currently only be pushed by @klardotsh as nobody else
has access to the Docker Hub org, but that can change in the future.

Dockerfile_base

@@ -0,0 +1,21 @@
+# vim: ft=dockerfile
+
+# Not using python:3.7 here because team-gcc-arm-embedded/ppa does not support
+# Ubuntu Cosmic or Debian Stretch, and Alpine, bizarrely, does not seem to
+# package GCC cross compilers
+FROM ubuntu:bionic
+
+# Set up PPAs we'll need for Python and for GCC ARM
+RUN apt-get update && apt-get install -y software-properties-common
+RUN add-apt-repository ppa:deadsnakes/ppa
+RUN add-apt-repository ppa:team-gcc-arm-embedded/ppa
+
+# Install Python
+RUN apt-get update && apt-get install -y python3.7 python3.7-dev build-essential pkg-config libffi-dev curl
+RUN curl https://bootstrap.pypa.io/get-pip.py | python3.7
+# Downgrade pip to work around https://github.com/pypa/pipenv/issues/2924
+RUN python3.7 -m pip install pip==18.0
+RUN python3.7 -m pip install pipenv==2018.7.1
+
+# Install KMK CI and/or build-time dependencies
+RUN apt-get install -y gcc-arm-embedded gettext ssh wget unzip rsync git locales libusb-dev