Pass file location to lifecycle commands (#173)

* Add test case for extending image and calling through to rsync

* Keep backup file location env var

* Add documentation

* Work against untared content in test

cmd/backup/exec.go

@@ -23,10 +23,14 @@ import (
 
 func (s *script) exec(containerRef string, command string) ([]byte, []byte, error) {
 	args, _ := argv.Argv(command, nil, nil)
+	commandEnv := []string{
+		fmt.Sprintf("COMMAND_RUNTIME_ARCHIVE_FILEPATH=%s", s.file),
+	}
 	execID, err := s.cli.ContainerExecCreate(context.Background(), containerRef, types.ExecConfig{
 		Cmd:          args[0],
 		AttachStdin:  true,
 		AttachStderr: true,
+		Env:          commandEnv,
 	})
 	if err != nil {
 		return nil, nil, fmt.Errorf("exec: error creating container exec: %w", err)