docker: correctly escape $BACKEND in the help output

docker: correctly escape $BACKEND in the help output

In Makefiles the $ must be escaped as $$ in shell uses.

Since 8a2390a4f47:

 $ make docker
     [...]
     NETWORK=1            Enable virtual network interface with default backend.
     NETWORK=ACKEND     Enable virtual network interface with ACKEND.

Once escaped:

 $ make docker
     [...]
     NETWORK=1            Enable virtual network interface with default backend.
     NETWORK=$BACKEND     Enable virtual network interface with $BACKEND.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-Id: <20171108024719.8389-1-f4bug@amsat.org>
Signed-off-by: Fam Zheng <famz@redhat.com>