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 <[email protected]>

Makefile

@@ -41,7 +41,7 @@ all: build
 
 build: 
 	@echo "Checking if Docker is available..."
-	@if command -v docker &> /dev/null ; then \
+	@if command -v docker >/dev/null 2>&1 ; then \
 		echo "Docker is available, building inside Docker container..."; \
 		$(MAKE) build-container; \
 	else \