Set development env vars in dev documentation correctly

Without this, the current dev setup documentation does not
work.

docs/contributing/dev-setup.md

@@ -24,6 +24,9 @@ The easiest & safest way to develop & test TLJH is with [Docker](https://www.doc
      --detach \
      --name=tljh-dev \
      --publish 12000:80 \
+     --env TLJH_BOOTSTRAP_DEV=yes \
+     --env TLJH_BOOTSTRAP_PIP_SPEC=/srv/src \
+     --env PATH=/opt/tljh/hub/bin:${PATH} \
      --mount type=bind,source="$(pwd)",target=/srv/src \
      tljh-systemd
    ```

integration-tests/Dockerfile

@@ -1,4 +1,4 @@
-# Systemd inside a Docker container, for CI only
+# Systemd inside a Docker container, for CI and local development
 ARG BASE_IMAGE=ubuntu:22.04
 FROM $BASE_IMAGE
 
@@ -28,9 +28,4 @@ RUN systemctl set-default multi-user.target
 
 STOPSIGNAL SIGRTMIN+3
 
-# Uncomment these lines for a development install
-# ENV TLJH_BOOTSTRAP_DEV=yes
-# ENV TLJH_BOOTSTRAP_PIP_SPEC=/srv/src
-# ENV PATH=/opt/tljh/hub/bin:${PATH}
-
 CMD ["/bin/bash", "-c", "exec /lib/systemd/systemd --log-target=journal 3>&1"]