Cleanup manpage container in build process

When running a `make manpages` the image `docker-manpage-dev` will be build and started to create
the man pages. But the container will not be deleted afterwards. So I propose to start it with
`docker run --rm ...`, otherwise we'll collect some trash.
```
$ docker ps -a
CONTAINER ID        IMAGE                COMMAND                  CREATED             STATUS                      PORTS               NAMES
e3d9f721d862        docker-manpage-dev   "man/generate.sh"        19 minutes ago      Exited (0) 16 minutes ago                       berserk_jang
5738a23fb682        docker-manpage-dev   "man/generate.sh"        9 hours ago         Exited (0) 9 hours ago                          boring_bartik
15490b5e63f7        docker-manpage-dev   "man/generate.sh"        10 hours ago        Exited (0) 10 hours ago                         lonely_joliot
```

Signed-off-by: Dieter Reuter <dieter.reuter@me.com>

Makefile

@@ -136,7 +136,7 @@ validate: build ## validate DCO, Seccomp profile generation, gofmt,\n./pkg/ isol
 
 manpages: ## Generate man pages from go source and markdown
 	docker build -t docker-manpage-dev -f "man/$(DOCKERFILE)" ./man
-	docker run \
+	docker run --rm \
 		-v $(PWD):/go/src/github.com/docker/docker/ \
 		docker-manpage-dev