dockertest.dockercmd: Print stdin when provided

It's hard to know, what happened while executing a command, when you
don't know the provided stdin. This patch appends `<<< "..."` after the
docker execute message in case it's non-zero. I used "%r" to make it
easier to discover, that it's not part of the executed command plus to
decrease size of the log.

Example:
Execute /usr/bin/docker -D run --name test_rvSj fedora bash  <<< 'ls /\nexit\n'
Execute /usr/bin/docker -D run --name test_rvSj fedora ls /

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