docker: correctly escape $BACKEND in the help output

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

Since 8a2390a4:

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

tests/docker/Makefile.include

@@ -109,7 +109,7 @@ docker:
 	@echo '    DEBUG=1              Stop and drop to shell in the created container'
 	@echo '                         before running the command.'
 	@echo '    NETWORK=1            Enable virtual network interface with default backend.'
-	@echo '    NETWORK=$BACKEND     Enable virtual network interface with $BACKEND.'
+	@echo '    NETWORK=$$BACKEND     Enable virtual network interface with $$BACKEND.'
 	@echo '    NOUSER               Define to disable adding current user to containers passwd.'
 	@echo '    NOCACHE=1            Ignore cache when build images.'
 	@echo '    EXECUTABLE=<path>    Include executable in image.'