Switch to static bash for etcd

- use bash-static and avoid dragging in all of debian-base
- use distroless as the base image
- the shell script needs a `cp` utility, add a go based one for just
  files (no support for directories!)
- Rework the calls to `mv` and recursive `cp` in the code
- we don't need to support windows in this container image
- the test case was slightly off as it was assuming that the old
  directory was copied into the new directory, but the desired
  functionality is that all files in the old directory should be in the
  new directory.

Signed-off-by: Davanum Srinivas <davanum@gmail.com>

cluster/images/etcd/migrate/data_dir_test.go

@@ -132,6 +132,10 @@ func TestBackup(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Failed to open data dir: %v", err)
 	}
+	_, err = os.Create(filepath.Join(path, "data-dir", "empty.txt"))
+	if err != nil {
+		t.Fatal(err)
+	}
 	err = d.Backup()
 	if err != nil {
 		t.Fatalf("Failed to backup data directory %s: %v", d.path, err)