Separate runc binary version from libcontainer version

Now that the dependency on runc (libcontaienr) code has been reduced
considerably, it is probbaly ok to cut the version dependency between
libcontainer and the runc binary that is supported.

This patch separates the runc binary version from the version of
libcontainer that is defined in go.mod, and updates the documentation
accordingly.

The RUNC_COMMIT variable in the install-runc script is renamed to
RUNC_VERSION to encourage using tagged versions, and the Dockerfile
in contrib is updated to allow building with a custom version.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

script/setup/install-runc

@@ -21,13 +21,14 @@
 set -eu -o pipefail
 
 function install_runc() {
-	RUNC_COMMIT=$(grep opencontainers/runc "$GOPATH"/src/github.com/containerd/containerd/go.mod | awk '{print $2}')
+	# When updating RUNC_VERSION, consider updating the runc module in go.mod as well
+	: "${RUNC_VERSION:=v1.0.0-rc93}"
 
 	TMPROOT=$(mktemp -d)
 	git clone https://github.com/opencontainers/runc.git "${TMPROOT}"/runc
 	pushd "${TMPROOT}"/runc
-	git checkout "${RUNC_COMMIT}"
-	make runc
+	git checkout "${RUNC_VERSION}"
+	make
 	make install
 	popd
 	rm -fR "${TMPROOT}"