wsk action get --save should work for docker actions

## Environment details:

* local deployment

## Steps to reproduce the issue:

1. create a docker action e.g. `wsk action create test --docker alpine:3.6 test.js`
2. get action with `--save` e.g. `wsk action get test --save`
3. result: `error: Cannot save Docker images`

## Provide the expected results and outputs:

```
ok: saved action code to ...
```

## Provide the actual results and outputs:

```
error: Cannot save Docker images
```

## Additional information you deem important:
* I'm not sure if there is a particular reason this code (not the docker image!) should not be savable?