Add docs for setting up a developer environment

Dockerfile

@@ -0,0 +1,28 @@
+# Systemd inside a Docker container, for CI only
+FROM ubuntu:18.04
+
+RUN apt-get update --yes
+
+RUN apt-get install --yes systemd curl git
+
+# Kill all the things we don't need
+RUN find /etc/systemd/system \
+    /lib/systemd/system \
+    -path '*.wants/*' \
+    -not -name '*journald*' \
+    -not -name '*systemd-tmpfiles*' \
+    -not -name '*systemd-user-sessions*' \
+    -exec rm \{} \;
+
+RUN mkdir -p /etc/sudoers.d
+
+RUN systemctl set-default multi-user.target
+
+STOPSIGNAL SIGRTMIN+3
+
+# Set up image to be useful out of the box for development & CI
+ENV TLJH_INSTALL_PIP_FLAGS="--no-cache-dir -e"
+ENV TLJH_INSTALL_PIP_SPEC=/srv/src
+ENV PATH=/opt/tljh/hub/bin:${PATH}
+
+CMD ["/bin/bash", "-c", "exec /sbin/init --log-target=journal 3>&1"]