Make it possible to override the volume mounts and shell for the dev container

[wk8]

- What I did

This patch provides some way for non-Linux devs to work on this repo without
suffering too much from the notorious terrible I/O perf of mounted volumes (eg
on OS X).

- How I did it

We now allow to override the volume mounts and shell for the dev container
through the DOCKER_CLI_MOUNTS and DOCKER_CLI_SHELL env variables.
We also allow setting the dev container name through the
DOCKER_CLI_CONTAINER_NAME env var.

The motivation for allowing overriding the volume mounts is the same as for
moby/moby#37845, namely that I/O perf on native mounted
disks on non-Linux (notably Mac OS) is just terrible, thus making it a real
pain to develop: one has to choose between re-building the image after every
single change (eg to run a test) or just work directly inside the same
container (eg with vim, but even then one would have to re-configure their dev
container every time it gets destroyed - containers, after all, are not
supposed to be long-lived).

Allowing to override DOCKER_CLI_MOUNTS makes it easy for everyone
to decide what their volume/syncing strategy is; for example
one can choose to use docker-sync.

As for the shell, it's nice to be able to use bash instead of the more
bare-bones ash if preferred.

Finally, being able to name the container can come in handy for easier
scripting on the host.

This patch won't change anything for anyone who doesn't set these env variables
in their environment.

- How to verify it

The output of:

make -f docker.Makefile -n shell

should be unchanged compared to what it was before this patch; but:

DOCKER_CLI_MOUNTS='-v test:/test' make -f docker.Makefile -n shell

should replace the -v flag from the command above with just
-v test:/test, and finally:

DOCKER_CLI_MOUNTS=' ' make -f docker.Makefile -n shell

should be devoid of any -v flags (besides the socket one).

All the make targets should be unchanged when not setting any of the
new env vars.

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

[codecov-io]

Codecov Report

Merging #1511 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1511   +/-   ##
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  Coverage   55.32%   55.32%           
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  Files         283      283           
  Lines       19330    19330           
=======================================
  Hits        10694    10694           
  Misses       7939     7939           
  Partials      697      697

[wk8]

Small review please? :)

[silvin-lubecki]

Sounds good to me, if it helps developing on the docker/cli repo 👍

[andrewhsu]

SGTM as well!

[wk8]

Thanks @andrewhsu and @silvin-lubecki for the review :) What's the process to get it merged at this point?

[cpuguy83]

LGTM

[wk8]

Thanks!