new "image" mount type

Add a new "image" mount type to `--mount`.  The source of the mount is
the name or ID of an image.  The destination is the path inside the
container.  Image mounts further support an optional `rw,readwrite`
parameter which if set to "true" will yield the mount writable inside
the container.  Note that no changes are propagated to the image mount
on the host (which in any case is read only).

Mounts are overlay mounts.  To support read-only overlay mounts, vendor
a non-release version of Buildah.

Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

vendor/github.com/prometheus/procfs/internal/util/sysreadfile.go

@@ -1,45 +0,0 @@
-// Copyright 2018 The Prometheus Authors
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-// http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-// +build linux,!appengine
-
-package util
-
-import (
-	"bytes"
-	"os"
-	"syscall"
-)
-
-// SysReadFile is a simplified ioutil.ReadFile that invokes syscall.Read directly.
-// https://github.com/prometheus/node_exporter/pull/728/files
-func SysReadFile(file string) (string, error) {
-	f, err := os.Open(file)
-	if err != nil {
-		return "", err
-	}
-	defer f.Close()
-
-	// On some machines, hwmon drivers are broken and return EAGAIN.  This causes
-	// Go's ioutil.ReadFile implementation to poll forever.
-	//
-	// Since we either want to read data or bail immediately, do the simplest
-	// possible read using syscall directly.
-	b := make([]byte, 128)
-	n, err := syscall.Read(int(f.Fd()), b)
-	if err != nil {
-		return "", err
-	}
-
-	return string(bytes.TrimSpace(b[:n])), nil
-}