This makes some small improvements to the makefile and development container:

Makefile: add missing help messages for some targets

Some targets didn't have a help message. After this:

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$

Makefile: don't warn "outside container" for some targets

This change allows some make targets to be ran outside the dev-container for
easier discovery and use:

• make clean can be used on the host (as artifacts created from within the
  development container are usually stored on the host).

• make help was already allowed

• make dev and make shell are added to the regular Makefile, to make it
  easier to create and start the development container.

• When attempting to run make dev from within the development container, a
  message is printed, and the target is cancelled:

  root@docker-cli-dev$ make dev
  you are already in the dev container

The warning has also been tweaked slightly:

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)