Add support for using podman to util/docker_build.sh (#10819)

* add podman support to docker_build.sh script

* break out runtime into the RUNTIME variable
* allows RUNTIME to be set by the user
* decides on docker or podman if docker isn't avaible
* rewrote check for docker-machine to account only for docker runtime
* put --user arg into a variable only to be used with docker
  this is not needed with podman as podman maps the containers root id
  to the users id.

* add podman to getting_started_docker documentation

util/docker_build.sh

@@ -17,12 +17,27 @@ done
 if [ $# -gt 1 ]; then
 	errcho "$USAGE"
 	exit 1
-elif ! command -v docker >/dev/null 2>&1; then
-	errcho "Error: docker not found"
-	errcho "See https://docs.docker.com/install/#supported-platforms for installation instructions"
-	exit 2
 fi
 
+# Allow $RUNTIME to be overriden by the user as an environment variable
+# Else check if either docker or podman exit and set them as runtime
+# if none are found error out
+if [ -z "$RUNTIME" ]; then
+	if command -v docker >/dev/null 2>&1; then
+		RUNTIME="docker"
+	elif command -v podman >/dev/null 2>&1; then
+		RUNTIME="podman"
+	else
+		errcho "Error: no compatible container runtime found."
+		errcho "Either podman or docker are required."
+		errcho "See https://podman.io/getting-started/installation"
+		errcho "or https://docs.docker.com/install/#supported-platforms"
+		errcho "for installation instructions."
+		exit 2
+	fi
+fi
+	
+
 # Determine arguments
 if [ $# -eq 0 ]; then
 	printf "keyboard=" && read -r keyboard
@@ -41,20 +56,26 @@ if [ -z "$keyboard" ]; then
 	keyboard=all
 fi
 if [ -n "$target" ]; then
-	if [ "$(uname)" = "Linux" ] || docker-machine active >/dev/null 2>&1; then
-		usb_args="--privileged -v /dev:/dev"
-	else
+	# IF we are using docker on non Linux and docker-machine isn't working print an error
+	# ELSE set usb_args
+	if [ ! "$(uname)" = "Linux" ] && [ "$RUNTIME" = "docker" ] && ! docker-machine active >/dev/null 2>&1; then
 		errcho "Error: target requires docker-machine to work on your platform"
 		errcho "See http://gw.tnode.com/docker/docker-machine-with-usb-support-on-windows-macos"
 		errcho "Consider flashing with QMK Toolbox (https://github.com/qmk/qmk_toolbox) instead"
 		exit 3
+	else
+		usb_args="--privileged -v /dev:/dev"
 	fi
 fi
 dir=$(pwd -W 2>/dev/null) || dir=$PWD  # Use Windows path if on Windows
 
+if [ "$RUNTIME" = "docker" ]; then
+	uid_arg="--user $(id -u):$(id -g)"
+fi
+
 # Run container and build firmware
-docker run --rm -it $usb_args \
-	--user $(id -u):$(id -g) \
+"$RUNTIME" run --rm -it $usb_args \
+	$uid_arg \
 	-w /qmk_firmware \
 	-v "$dir":/qmk_firmware \
 	-e ALT_GET_KEYBOARDS=true \