Revamping Travis and Docker setup introducing a `Makefile`.
The idea is to have a single entry point via `make` and reproducible
builds via Docker.
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.

Revamping Travis and Docker setup introducing a `Makefile`.
The idea is to have a single entry point via `make` and reproducible
builds via Docker.
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.

Revamping Travis and Docker setup introducing a `Makefile`.
The idea is to have a single entry point via `make` and reproducible
builds via Docker.
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.

Revamping Travis and Docker setup introducing a `Makefile`.
The idea is to have a single entry point via `make` and reproducible
builds via Docker.
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.

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.

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.

Travis CI revamp part 1, refs #2008