Update Docker image doc re DB path
Update Docker image doc re DB path

... without masking other data.

As [@dincho
mentioned](https://github.com/aeternity/epoch/pull/632/files#r164061734),
other data is stored in `data` inside the container ...

```
$ ls -d data/*
data/aecore                     data/epoch_config_schema.json   data/mnesia
```

... hence may be hidden to the node.  Hence use more specific path
than `data`.

Docker `-v` behaviour as [commented by
@dincho](https://github.com/aeternity/epoch/pull/632/files#r164081395):

> When bind mount is used as in `-v
> ~/.aeternity/data:/home/epoch/node/data` if there are any
> files/directories in `/home/epoch/node/data` in the container, they
> would become unaccessible.

> Example/Test:

  ```
$ docker run -it --rm --entrypoint=bash aetrnty/epoch
epoch@8f8ac1e8959b:~/node$ ls data/
aecore  epoch_config_schema.json

$ mkdir /tmp/.aeternity && touch /tmp/.aeternity/blah
$ docker run -it --rm -v /tmp/.aeternity:/home/epoch/node/data --entrypoint=bash aetrnty/epoch
epoch@61f6fa7c7dae:~/node$ ls data/
blah
```

aeternity/epoch,aeternity/epoch,aeternity/epoch,aeternity/epoch