Add docker build testing to Travis

[abitrolly]

As a followup to #798 I decided to test if Docker build is still operational, continuously.

[codecov-io]

Codecov Report

Merging #799 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #799   +/-   ##
=======================================
  Coverage   96.87%   96.87%           
=======================================
  Files          59       59           
  Lines        3141     3141           
  Branches      401      401           
=======================================
  Hits         3043     3043           
  Misses         59       59           
  Partials       39       39

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[Zlopez]

Dockerfile is really outdated, so I'm not sure if the Anitya in docker will run at all.

[abitrolly]

@pypingou it is not ready yet. I am going to make proper stages, so the test will fail faster on improper linting etc.

[abitrolly]

@pypingou ready to be merged.

[Zlopez]

@abitrolly Could you squash the commits and add news file?
See https://anitya.readthedocs.io/en/stable/contributing.html#release-notes

[abitrolly]

@Zlopez done.

[Zlopez]

This looks much better in Travis. Thanks for your work :-)

[abitrolly]

Not sure what codecov is waiting for. Build seems to be processed successfully https://codecov.io/gh/release-monitoring/anitya/commit/48b446a2261608c5aee6c10bcc316c00abae2be4

Reported upstream https://community.codecov.io/t/codecov-status-stuck-at-waiting-for-status-to-be-reported/406