build: Use master of manifest-tool

The PR to enable notary signing has been merged. Also update
documentation as `got get` should now also work.

Signed-off-by: Rolf Neugebauer <rolf.neugebauer@docker.com>

Makefile

@@ -43,8 +43,8 @@ tmp_rtf_bin.tar: Makefile
 	docker run --rm --log-driver=none -e http_proxy=$(http_proxy) -e https_proxy=$(https_proxy) $(CROSS) $(GO_COMPILE) --clone-path github.com/linuxkit/rtf --clone https://github.com/linuxkit/rtf.git --commit $(RTF_COMMIT) --package github.com/linuxkit/rtf --ldflags "-X $(RTF_CMD).GitCommit=$(RTF_COMMIT) -X $(RTF_CMD).Version=$(RTF_VERSION)" -o bin/rtf > $@
 
 # Manifest tool for multi-arch images
-MT_COMMIT=186e7752e8032756bb263b830451f44e5176864f
-MT_REPO=https://github.com/rn/manifest-tool
+MT_COMMIT=bfbd11963b8e0eb5f6e400afaebeaf39820b4e90
+MT_REPO=https://github.com/estesp/manifest-tool
 bin/manifest-tool: tmp_mt_bin.tar | bin
 	tar xf $<
 	rm $<

docs/packages.md

@@ -20,8 +20,10 @@ Before you can build packages you need:
 - Docker version 17.06 or newer. If you are on a Mac you also need
   `docker-credential-osxkeychain.bin`, which comes with Docker for Mac.
 - `make`, `notary`, `base64`, `jq`, and `expect`
-- A custom copy of `manifest-tool` which you can build with `make
-  bin/manifest-tool`. `manifest-tool` must be in your path.
+- A *recent* version of `manifest-tool` which you can build with `make
+  bin/manifest-tool`, or `go get github.com:estesp/manifest-tool`, or
+  via the LinuxKit homebrew tap with `brew install --HEAD
+  manifest-tool`. `manifest-tool` must be in your path.
 
 Further, when building packages you need to be logged into hub with
 `docker login` as some of the tooling extracts your hub credentials