Add arm64 to releases

This moves all the release builds into a Dockerfile which is a bit
cleaner for setting up our build environment.

Non-linux/amd64 builds are cross-compiled.
Currently onlinux linux/amd64, linux/arm64, and windows/amd64 are
supported, but is easy to add more, provided their is a cross-compile
toolchain available for it.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

Makefile

@@ -30,7 +30,7 @@ MANDIR        ?= $(DATADIR)/man
 TEST_IMAGE_LIST ?=
 
 # Used to populate variables in version package.
-VERSION=$(shell git describe --match 'v[0-9]*' --dirty='.m' --always)
+VERSION ?= $(shell git describe --match 'v[0-9]*' --dirty='.m' --always)
 REVISION=$(shell git rev-parse HEAD)$(shell if ! git diff --no-ext-diff --quiet --exit-code; then echo .m; fi)
 PACKAGE=github.com/containerd/containerd
 SHIM_CGO_ENABLED ?= 0
@@ -271,6 +271,7 @@ install-man: man
 	@echo "$(WHALE) $@"
 	$(foreach manpage,$(addprefix man/,$(MANPAGES)), $(call installmanpage,$(manpage),$(subst .,,$(suffix $(manpage))),$(notdir $(manpage))))
 
+
 releases/$(RELEASE).tar.gz: $(BINARIES)
 	@echo "$(WHALE) $@"
 	@rm -rf releases/$(RELEASE) releases/$(RELEASE).tar.gz