docker: Fix $DOCKER_USER (#2057)

We do not try renaming $HOME anymore as there is no good way
to do it.

We also only try to convert if the user hasn't been changed.

Finally I added usage to the docker docs in install.md

Closes #2056

ci/release-image/Dockerfile

@@ -39,6 +39,9 @@ COPY ci/release-image/entrypoint.sh /usr/bin/entrypoint.sh
 RUN dpkg -i /tmp/code-server*$(dpkg --print-architecture).deb && rm /tmp/code-server*.deb
 
 EXPOSE 8080
-USER coder
+# This way, if someone sets $DOCKER_USER, docker-exec will still work as
+# the uid will remain the same. note: only relevant if -u isn't passed to
+# docker-run.
+USER 1000
 WORKDIR /home/coder
 ENTRYPOINT ["/usr/bin/entrypoint.sh", "--bind-addr", "0.0.0.0:8080", "."]