Make: allow specifying dir owner

this is useful if you need to run docker with sudo because you're not in
docker group

Makefile

@@ -4,6 +4,7 @@ PORT := 8000
 VENV := venv
 CONFIG := dev_config.py
 PY := python
+OWNER := ${USER}
 
 docker-build:
 	$(DOCKERC) build \
@@ -22,8 +23,8 @@ docker-build-liquidsoap:
 docker-build-techrec:
 	$(DOCKERC) build \
 		--no-cache \
-		--build-arg=hostgid=$(shell id -g) \
-		--build-arg=hostuid=$(shell id -u) \
+		--build-arg=hostgid=$(shell id -g ${OWNER}) \
+		--build-arg=hostuid=$(shell id -u ${OWNER}) \
 		techrec
 
 docker-stop: