Only accepts dockerfile named Dockerfile 

## Current Behavior
If I name my dockerfile something other than 'Dockerfile' (say 'db.Dockerfile'), during `app deploy` it will give me this error: _"unable to prepare context: context must be a directory"_. 

```bash
$ ecs-preview app init --dockerfile "db.Dockerfile"
$ ecs-preview app deploy # throws error
```

## Expected Behavior
Not throwing this error
