fix docker permissions issue due to mounting volume

see: moby/moby#7198

makefile

@@ -21,11 +21,6 @@ build-pure-on:
 		--tag=pure-on-fish-${FISH_VERSION} \
 		./
 
-.PHONY: test-pure-on
-test-pure-on: export CMD=fishtape tests/*.test.fish  # can be overriden by user
-test-pure-on:
-	@$(MAKE) dev-pure-on  # ${CMD} is passed along
-
 .PHONY: dev-pure-on
 dev-pure-on: CMD?=fish
 dev-pure-on:
@@ -36,3 +31,14 @@ dev-pure-on:
 		--tty \
 		--volume=$$(pwd):/tmp/.pure/ \
 		pure-on-fish-${FISH_VERSION} "${CMD}"
+
+# Don't override COPY directive as `--volume` doesnt play nice with Travis
+.PHONY: test-pure-on
+test-pure-on: export CMD=fishtape tests/*.test.fish  # can be overriden by user
+test-pure-on:
+	docker run \
+		--name run-pure-on-${FISH_VERSION} \
+		--rm \
+		--interactive \
+		--tty \
+		pure-on-fish-${FISH_VERSION} "${CMD}"