Alpine image: allows CMD override to shell when a non Traefik command is provided

[dduportal]

Related to docker-library/official-images#6106 (comment)

TL;DR; the "official Docker image test suite" run by Docker requires the CMD to be overridden when an image has a shell (as the Alpine Traefik).
With v2.0-alpha6, the Traefik CLI accepts non flags options, which leads to issues.

This change is only alpine, and delegate to the main shell the user-provided CMD if it is not a command recognized by traefik.

So the following exemple works:

# print Traefik version
docker run --rm traefik:<tag> version
# print Traefik help about healthcheck Traefik's command
docker run --rm traefik:<tag> healthcheck --version
# print "Hello world-<random number>-<random number>" with a message in the log that switched to shell
docker run --rm traefik:<tag> echo Hello world-$RANDOM-$RANDOM

Important: works with 1.7.12, but for 2.0, it requires traefik/traefik#4977 to be merged!

[ldez]

LGTM