Revamping Travis and Docker setup introducing a `Makefile`.
The idea is to move the CI complexity from .travis.yml to `Makefile`.
That makes a single entry point via `make` command and reproducible
builds via Docker.
It makes it easy to run some commands outside docker, such as:
```sh
make testapps/python3/armeabi-v7a
```
Or the same command inside docker:
```sh
make docker/run/make/testapps/python3/armeabi-v7a
```
This pull request also starts introducing some docker layer cache
optimization as needed by kivy#2009 to speed up docker pull/push and
rebuilds from cache.
It also introduces other Docker images good practices like ordering
dependencies alphabetically or always enforcing `apt update` prior
install, refs:
https://docs.docker.com/develop/develop-images/dockerfile_best-practices/
Subsequent pull requests would simplify the process furthermore and
leverage the cache to speed up builds.

The `docker build` is now running alongside the `tox` testing as part of
the `pre checks` stage. It also fully leverages the docker image cache.
The newly built image is pushed to be available for subsequent build
stages and pull requests.
Two environment variables `DOCKER_USERNAME` and `DOCKER_PASSWORD` got
added to the Travis UI for making image pushing possible. Both `push`
and `pull` features fail silently with `|| true` as they should be
optional for a valid build.
`TRAVIS_PULL_REQUEST` and `TRAVIS_BRANCH` are built-in Travis variables.
Note that we're passing variables to `Makefile` explicitly on `make`
call rather than using the `--environment-overrides` flag. While it's
more verbose, it makes it easier to follow what's being used and when.
With this change, `Python 3 arm64-v8a` the longest `testapps` build that
was taking ~26 minutes to complete is now taking ~20 minutes when
hitting docker layers cache (most of the time). The shorter build
`Rebuild updated recipes` was taking ~9 minutes and is now down to ~3
minutes.
Closes kivy#2009