Get MPY cross compiler into the Docker image

.circleci/config.yml

@@ -1,3 +1,4 @@
+---
 version: 2
 jobs:
   test:
@@ -5,8 +6,8 @@ jobs:
       - image: 'kmkfw/base'
 
     environment:
-        KMK_TEST: 1
-        PIPENV_VENV_IN_PROJECT: 1
+      KMK_TEST: 1
+      PIPENV_VENV_IN_PROJECT: 1
 
     steps:
       - checkout

Dockerfile_base

@@ -1,9 +1,22 @@
 # 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 python:3.7-alpine
 
-RUN apk update && apk add alpine-sdk curl wget openssh rsync git git-lfs
+RUN mkdir -p /app
+WORKDIR /app
+
+RUN apk update && apk add alpine-sdk curl gettext git git-lfs openssh rsync wget
+
 RUN pip install pipenv
+
+### Get a local copy of CircuitPython and its dependencies
+# Our absolute baseline is 4.0.0, which (as of writing) shares MPY compat
+# with all future versions. Our baseline will need to update as MPY compat
+# changes
+RUN git clone --branch 4.0.0 --depth 1 https://github.com/adafruit/CircuitPython /opt/circuitpython
+RUN git -C /opt/circuitpython submodule update --init
+
+### Build the MPY compiler
+RUN make -C /opt/circuitpython/mpy-cross
+
+ENV PATH=/opt/circuitpython/mpy-cross:${PATH}