Bash scripts; use double brackets, fix bare variables, add quotes

These scripts explicitly use Bash, so we should be able to use
`[[` instead of `[` (which seems to be recommended).

Also added curly brackets to some bare variables, and quoted some paths.

This makes my IDE a bit more silent :-)

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

hack/dockerfile/install/runc.installer

@@ -25,6 +25,6 @@ install_runc() {
 		target="$1"
 	fi
 	make BUILDTAGS="$RUNC_BUILDTAGS" "$target"
-	mkdir -p ${PREFIX}
-	cp runc ${PREFIX}/runc
+	mkdir -p "${PREFIX}"
+	cp runc "${PREFIX}/runc"
 }