docker: only build dockerfile's on changes

Makefile

@@ -4,7 +4,7 @@ check:
 	-@./bin/setup
 
 SUBDIRS := $(dir $(wildcard */Makefile))
-TARGETS := install clean test test-ci lint lint-ci build push # whatever else, but must not contain '/'
+TARGETS := install clean test test-ci lint lint-ci # whatever else, but must not contain '/'
 
 # foo/.all bar/.all foo/.clean bar/.clean
 SUBDIRS_TARGETS := \
@@ -23,3 +23,29 @@ $(TARGETS): %: $(addsuffix %,$(SUBDIRS))
 #   $(@F:.%=%) is just all
 $(SUBDIRS_TARGETS):
 	$(MAKE) --no-print-directory -C $(@D) $(@F:.%=%)
+
+DIFF := $(lastword $(subst /, ,${CIRCLE_COMPARE_URL}))
+CHANGES := $(filter $(dir $(shell git diff --name-only $(DIFF))), $(SUBDIRS))
+.PHONY: diff
+diff:
+	echo $(CHANGES)
+
+BUILDS := build push
+
+# foo/.all bar/.all foo/.clean bar/.clean
+BUILDS_TARGETS := \
+	$(foreach t,$(BUILDS),$(addsuffix $t,$(CHANGES)))
+
+.PHONY: $(BUILDS) $(BUILDS_TARGETS)
+
+# static pattern rule, expands into:
+# all clean: %: foo/.% bar/.%
+$(BUILDS): %: $(addsuffix %,$(CHANGES))
+	@echo 'Done "$*" target'
+
+# here, for foo/.all:
+#   $(@D) is foo
+#   $(@F) is .all, with leading period
+#   $(@F:.%=%) is just all
+$(BUILDS_TARGETS):
+	$(MAKE) --no-print-directory -C $(@D) $(@F:.%=%)