Bash is installed already, so we might as well use it as a default.
This also removes the DOCKER_CLI_SHELL variable, which was added
in b039db985a to allow using bash
instead of (a)sh. Now that Bash is the default, there should be
no need to override, and we can keep things simple.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>