Publish docker image

[Jamstah]

Add a deploy stage to the travis build to publish a latest docker image to a registry for master and releases.

There seem to be a lot of webssh images on docker, it would be nice to have an official, regularly updated one to rely on..

If you agree that its a good idea, this PR would also require that you create a dockerhub account, and add the DOCKER_USERNAME and DOCKER_PASSWORD values to travis to be able to push the image.

[codecov-io]

Codecov Report

Merging #213 (41fe43b) into master (2f0d580) will decrease coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #213      +/-   ##
==========================================
- Coverage   94.20%   94.08%   -0.13%     
==========================================
  Files           8        8              
  Lines         829      829              
  Branches      142      142              
==========================================
- Hits          781      780       -1     
  Misses         32       32              
- Partials       16       17       +1     

Impacted Files	Coverage Δ
webssh/handler.py	96.67% <0.00%> (-0.24%)	⬇️

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[huashengdun]

I think it is a good idea to publish an "official" docker image for this app.
But is it too much to build and publish the docker image on every commit?
I would rather consider doing it on every release.

[Jamstah]

I would argue that having a bleeding edge latest isn't a bad thing, but I don't mind, its your project :)

Having latest track master also means you don't have to work out how to push the right latest image tag when you push a new release.

To stop it deploying latest, just remove that provider from the .travis. As there is other set up to do on the project (that I can't do because I'm not an admin), I'll leave that up to you. Happy to help out if needed.