add Dockerfile

[OBrink]

Hey there!

As discussed in #31 with @rccc and @shohnmccullough, it might be sensible to provide a Docker image to address the dependency issues that have been introduced with the new chemical entity recognition system in version 2.1 (see #13, #24, #29, #30, #31).

All I have done here is add a Dockerfile that specifies how to generate the image. This way, people can use CDE 2.1.2 on any operating system.

I have uploaded the image here.

[rccc]

@OBrink

Thanks a lot !

For those who will use this image do not forget to add the tag :

docker pull obrink/chemdataextractor:2.1.2