docker: add shell command to entrypoint

Makefile

@@ -233,6 +233,3 @@ docker-push:
 
 docker-pull:
 	env docker pull glenux/teaching-boilerplate:latest
-
-shell:
-	$(SHELL) -c "exec $${SHELL}"

docker/entrypoint.sh

@@ -21,4 +21,9 @@ if [ -d "_build" ]; then
 	chown -R "$EXT_UID:$EXT_GID" _build
 fi
 
+if [ "$1" = "shell" ]; then
+	exec bash
+else
 	exec gosu "$EXT_UID:$EXT_GID" make "$@"
+fi
+