Build and upload Swagger Codegen CLI Docker image on release

[arnested]

PR checklist

• Read the contribution guildelines.
• Ran the shell/batch script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates)
• Filed the PR against the correct branch: master for non-breaking changes and 2.3.0 branch for breaking (non-backward compatible) changes.

Description of the PR

Follow-up on the improved Docker stuff in #2945 this pull request adds building and uploading a Swagger Codegen CLI Docker image to Docker Hub on releases.

[wing328]

Seems like there are issues with job running for too long and got timed out: https://travis-ci.org/swagger-api/swagger-codegen/builds/206925853

[arnested]

Yes, I did the environment stuff in .travis.yml wrong (adding the two variables as two entries under env: which triggers a build for each entry instead of adding the new environment variable to the existing entry).

A fix is pushed. This time the build hopefully should not timeout...

[arnested]

Hmmm... The build aborts with

The log length has exceeded the limit of 4 MB (this usually means that the test suite is raising the same exception over and over).

The job has been terminated

The long log is from mvh downloading everything. I guess the initial faulty build has wiped some travis cache of the downloads for this pull request (since this doesn't appear to be a problem for other pull requests).

I'll close this pull request anf create a new (#4912) to see if this will fix the problem.

[arnested]

The new pull request wasn't enough to cut down on the logging output. But running mvn in betch mode was. So I changed that as well in the other pull request.

[wing328]

Ok. One suggestion is to add a 1-liner comment explaining the new tests.

[arnested]

I added some comments in the other pull request.