add ENV PATH so downstream Dockerfiles can expect to find anaconda binaries in PATH

Hi!

I'm new to Docker.  Please forgive me if I'm missing something here.  What do you think about adding the following line to the Dockerfile?

```
ENV PATH /opt/anaconda/bin:$PATH
```

My reasoning for this is that if decide to have my app inherit ```FROM continuumio/miniconda``` in a separate Dockerfile, I see that conda and python binaries are not immediately available in the PATH without first initializing a login shell.  

For instance, shouldn't

```
 $ docker run continuumio/miniconda env | grep PATH
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
```

be the same as this?

```
 $ docker run continuumio/miniconda bash -lc 'env' | grep PATH
PATH=/opt/anaconda/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
```

This pull request results in:

```
 $ docker run adgaudio/miniconda env | grep PATH
PATH=/opt/anaconda/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
```