fix(Makefile): delete build container and container runtime

changes:
- delete build container after any container step
- define container runtime over variable

Makefile

@@ -10,6 +10,8 @@ GID?=$(shell id --group)
 # value is taken.
 VERSION:=$(or ${TRAVIS_TAG}, ${TRAVIS_TAG}, $(shell git rev-parse --short HEAD)-git)
 
+CONTAINER_RUNTIME?=$(shell which docker)
+
 GOARCH?=amd64
 GOOS?=linux
 
@@ -33,8 +35,9 @@ container-test:
 	$(MAKE) container-run COMMAND=test
 
 container-run:
-	docker run \
+	${CONTAINER_RUNTIME} run \
 		--volume ${PWD}:/workspace \
+		--rm \
 		volkerraschek/build-image:latest\
 			make ${COMMAND} UID=${UID} GID=${GID} VERSION=${VERSION} GOOS=${GOOS} GOARCH=${GOARCH}