[ci-suggested-fix] Using docker hub build hook

[edify42]

WHAT

Be able to inject --build-arg variables from the environment.

WHY

A colleague and myself noticed how the docker images were being pushed on docker hub for this repo from the README.md link. We wanted newer images that could be built directly from the master branch here.

Instead I'd like to propose a number of buildable docker images repositories (e.g. /metricbeat and /filebeat) each pointing to the master branch of this repo.

Within the Docker hub build configuration section, you can declare the environment variables like so:

Variables such as BEAT_DOCKER_IMAGE and BEAT_NAME would be defined on the environment. The IMAGE_TAG would need ideally be semver'd in some way...

Relates to the builds of images from this issue

[codecov-io]

Codecov Report

Merging #60 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #60   +/-   ##
=======================================
  Coverage   64.96%   64.96%           
=======================================
  Files           4        4           
  Lines         137      137           
=======================================
  Hits           89       89           
  Misses         42       42           
  Partials        6        6

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[mumoshu]

Wow! I've never realized that Docker Hub had the build env support! I'll review this asap. Thanks for your contribution 👍