Add a (make vendor) target, primarily to preserve c/image documentation

The goal is to include the c/image documentation in a skopeo release,
so that RPMs and other distribution mechanisms can ship the c/image
documentation without having to create a separate package for c/image
(which would not otherwise be needed because it is vendored in users).

So, unify the updates of the "vendor" subdirectory as (make vendor),
and document it in README.md.  Also drop hack/vendor.sh, we neither
use nor document it, so updating it as well seems pointless.

Makefile

@@ -135,3 +135,6 @@ validate-local:
 
 test-unit-local:
 	$(GPGME_ENV) $(GO) test -tags "$(BUILDTAGS)" $$($(GO) list -tags "$(BUILDTAGS)" -e ./... | grep -v '^github\.com/projectatomic/skopeo/\(integration\|vendor/.*\)$$')
+
+vendor: vendor.conf
+	vndr -whitelist '^github.com/containers/image/docs/.*'

README.md

@@ -224,30 +224,30 @@ CONTRIBUTING
 
 ### Dependencies management
 
-`skopeo` uses [`vndr`](https://github.com/LK4D4/vndr) for dependencies management.
+Make sure [`vndr`](https://github.com/LK4D4/vndr) is installed.
 
 In order to add a new dependency to this project:
 
 - add a new line to `vendor.conf` according to `vndr` rules (e.g. `github.com/pkg/errors master`)
-- run `vndr github.com/pkg/errors`
+- run `make vendor`
 
 In order to update an existing dependency:
 
 - update the relevant dependency line in `vendor.conf`
-- run `vndr github.com/pkg/errors`
+- run `make vendor`
 
 When new PRs for [containers/image](https://github.com/containers/image) break `skopeo` (i.e. `containers/image` tests fail in `make test-skopeo`):
 
 - create out a new branch in your `skopeo` checkout and switch to it
 - update `vendor.conf`. Find out the `containers/image` dependency; update it to vendor from your own branch and your own repository fork (e.g. `github.com/containers/image my-branch https://github.com/runcom/image`)
-- run `vndr github.com/containers/image`
+- run `make vendor`
 - make any other necessary changes in the skopeo repo (e.g. add other dependencies now requied by `containers/image`, or update skopeo for changed `containers/image` API)
 - optionally add new integration tests to the skopeo repo
 - submit the resulting branch as a skopeo PR, marked “DO NOT MERGE”
 - iterate until tests pass and the PR is reviewed
 - then the original `containers/image` PR can be merged, disregarding its `make test-skopeo` failure
 - as soon as possible after that, in the skopeo PR, restore the `containers/image` line in `vendor.conf` to use `containers/image:master`
-- run `vndr github.com/containers/image`
+- run `make vendor`
 - update the skopeo PR with the result, drop the “DO NOT MERGE” marking
 - after tests complete succcesfully again, merge the skopeo PR
 

hack/vendor.sh

@@ -1,15 +0,0 @@
-#!/bin/bash
-
-# This file is just wrapper around vndr (github.com/LK4D4/vndr) tool.
-# For updating dependencies you should change `vendor.conf` file in root of the
-# project. Please refer to https://github.com/LK4D4/vndr/blob/master/README.md for
-# vndr usage.
-
-set -e
-
-if ! hash vndr; then
-	echo "Please install vndr with \"go get github.com/LK4D4/vndr\" and put it in your \$GOPATH"
-	exit 1
-fi
-
-vndr "$@"