Dockerfile.dev: set prompt, add nano and jq, and set MOTD

This makes some minor improvments to the dev container:

- add `nano` as an alternative to `vi` to help beginning contributors, or people
  that don't remember how to quit `vi` ;-)
- add `jq` as it's often handy to have available to debug JSON output.
- set a custom prompt to make it clearer that the user is in a container
- add a short MOTD that shows that the user is in a container, and a pointer
  to 'make help' (we can add more help/instructions to this MOTD in future).

Before this patch:

    make -f docker.Makefile dev
    ...
    bash-5.1#

With this patch:

    make -f docker.Makefile dev
    ...

    You are now in a development container. Run 'make help' to learn about
    available make targets.

    root@docker-cli-dev$

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

dockerfiles/Dockerfile.dev

@@ -29,8 +29,12 @@ RUN  apk add --no-cache \
     ca-certificates \
     coreutils \
     curl \
-    git
+    git \
+    jq \
+    nano
 
+RUN echo -e "\nYou are now in a development container. Run '\e\033[1mmake help\e\033[0m' to learn about\navailable make targets.\n" > /etc/motd \
+ && echo -e "cat /etc/motd\nPS1=\"\e[0;32m\u@docker-cli-dev\\$ \e[0m\"" >> /root/.bashrc
 CMD bash
 ENV DISABLE_WARN_OUTSIDE_CONTAINER=1
 ENV PATH=$PATH:/go/src/github.com/docker/cli/build