dockertest/environment: Action required configuration

When the description starts with `!` it's rendered as **emphasized text**.
This can be used to define configs, which needs to be configured.

As an example docker_cli/build test config was rewriten using this
feature. Although I enabled the defaults in order to let the test work
by default. The user is responsible to change the locations in case it's
computers don't have access to the internet.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>