Specify explicit version of the base image

[osvso]

Currently the Dockerfile is based on latest ubuntu image FROM ubuntu and therefore may become unstable at some point in time. Please consider changing to specific verified version of ubuntu ex. ubuntu:16.04.

[tracer0tong]

Accordingly documentation on Dockerhub for ubuntu:latest container:

The ubuntu:latest tag points to the "latest LTS", since that's the version recommended for general use.

I'm not sure that it's so unsafe, because usually I'm trying to test container before pushing to Dockerhub. Do you have some other concerns?

[osvso]

Yes, my only concern is that the underlaying ubuntu changes in between versions and may require some additional repositories or libraries in order to get the emulator working. If someone will clone you repo and build the image locally he may find it not working, which shouldn't be the case if the image is pulled from docker repo and tested, as you mentioned, up-front.

[tracer0tong]

@nightscape what do you think about it?