Instead of running the web service as `root` and the CLI service
as `33:33`, we should be running them as the host user. By
doing this we ensure ownership parity in mounted folders
on platforms where having different owners would result
in permission issues.

With the changes in this pull request we
no longer need to do this.

This will ensure things like `wp plugin install` have a directory to
use as a cache. Anything else can now refer to the home directory
for the same purpose.

Some of our `docker-compose run` actions weren't removing the
container after being executed. This was breaking `wp-env destroy`
becuase of the volumes that were in-use.

By defining an `image` property in our `docker-compose.yml` we
were replacing the corresponding image locally after building. This
meant `tests-wordpress` would have the same image as `wordpress`
as long as the PHP versions matched. This only worked on accident,
and as a consequence, also replaced the local `wordpress` image.

This commit isolates all of the `build` operations so they don't replace
the images. This also adds support so that `destroy` will remove
them.

Co-authored-by: Noah Allen <noahtallen@gmail.com>

Debian stretch was just removed from deb.debian.org and moved
to the archive repository. This commit changes the sources
so that, on stretch, the apt-get update won't break.