Fix Makefile

Makefile

-all:
-	$(MAKE) -C 15.2
-	$(MAKE) -C 15.3
+TARGETS := $(patsubst %/Dockerfile,%,$(wildcard */Dockerfile))
+PUSH_TARGETS := $(patsubst %,%-push,$(TARGETS))
 
-push:
-	$(MAKE) -C 15.2 push
-	$(MAKE) -C 15.3 push
+.PHONY: $(TARGETS)
+
+all: $(TARGETS)
+
+$(TARGETS):
+	$(MAKE) -C $@
+
+$(PUSH_TARGETS):
+	$(MAKE) -C $@ push