Vendor all dependencies, add docker build to makefile.

Vendor and store all dependencies. This allows us to have a makefile which
builds to a minimized docker image.

vendor/github.com/prometheus/procfs/fs.go

@@ -0,0 +1,33 @@
+package procfs
+
+import (
+	"fmt"
+	"os"
+	"path"
+)
+
+// FS represents the pseudo-filesystem proc, which provides an interface to
+// kernel data structures.
+type FS string
+
+// DefaultMountPoint is the common mount point of the proc filesystem.
+const DefaultMountPoint = "/proc"
+
+// NewFS returns a new FS mounted under the given mountPoint. It will error
+// if the mount point can't be read.
+func NewFS(mountPoint string) (FS, error) {
+	info, err := os.Stat(mountPoint)
+	if err != nil {
+		return "", fmt.Errorf("could not read %s: %s", mountPoint, err)
+	}
+	if !info.IsDir() {
+		return "", fmt.Errorf("mount point %s is not a directory", mountPoint)
+	}
+
+	return FS(mountPoint), nil
+}
+
+// Path returns the path of the given subsystem relative to the procfs root.
+func (fs FS) Path(p ...string) string {
+	return path.Join(append([]string{string(fs)}, p...)...)
+}