Add an alternative method for keyboard discovery to speed up build (#6073)

* Add an alternative method for keyboard discovery to speed up build

* Chain MAKEFLAGS for docker_build.sh

* Slight improvement to number of items sent to sort

* Remove debug line

* Fix line escape

util/docker_build.sh

@@ -46,5 +46,11 @@ fi
 dir=$(pwd -W 2>/dev/null) || dir=$PWD  # Use Windows path if on Windows
 
 # Run container and build firmware
-docker run --rm -it $usb_args -w /qmk_firmware/ -v "$dir":/qmk_firmware qmkfm/base_container \
+docker run --rm -it $usb_args \
+    -w /qmk_firmware/ \
+	-v "$dir":/qmk_firmware \
+	-e ALT_GET_KEYBOARDS=true \
+	-e SKIP_GIT="$SKIP_GIT" \
+	-e MAKEFLAGS="$MAKEFLAGS" \
+	qmkfm/base_container \
 	make "$keyboard${keymap:+:$keymap}${target:+:$target}"