Detect Docker robustly

"&> /dev/null" works as a redirection from stdout/stderr to /dev/null
in Bash but not in POSIX shell (considered as an asynchronous execution
and the result of the "command" command cannot be retrieved).

As a result, it always assumes that Docker always exists.

This commit makes the redirection robust and portable (makes it possible
to detect "no Docker" condition correctly).

Signed-off-by: Tsukasa OI <research_trasio@irq.a4lg.com>