test/testvol: move Containerfile into testvol dir

I think it is confusion to have this Containerfile in the repo root. It
is used for the tests only so we should move it into the same dir.

Also adapt the Makefile target to use the new path and add the current
date as tag instead of using latest which can break CI easily when we
have to update the image.

Signed-off-by: Paul Holzinger <pholzing@redhat.com>

Makefile

@@ -227,9 +227,9 @@ test/checkseccomp/checkseccomp: $(wildcard test/checkseccomp/*.go)
 test/testvol/testvol: $(wildcard test/testvol/*.go)
 	$(GOCMD) build $(BUILDFLAGS) $(GO_LDFLAGS) '$(LDFLAGS_PODMAN)' -o $@ ./test/testvol
 
-.PHONY: volume-plugin-test-image
+.PHONY: volume-plugin-test-img
 volume-plugin-test-img:
-	podman build -t quay.io/libpod/volume-plugin-test-img -f Containerfile-testvol .
+	podman build -t quay.io/libpod/volume-plugin-test-img:$$(date +%Y%m%d) -f ./test/testvol/Containerfile .
 
 .PHONY: test/goecho/goecho
 test/goecho/goecho: $(wildcard test/goecho/*.go)