Makes it possible to build Android APKs directly from Docker.
Tested by building sucessfully both:
* kivy-garden/garden.zbarcam (20190303)
* AndreMiras/EtherollApp (20190217)
After building the image with:
```
docker build --tag=kivy/buildozer .
```
The following command was used to build the Android APK:
```
docker run \
--volume "$HOME/.buildozer":/home/user/.buildozer \
--volume "$(pwd)":/home/user/hostcwd \
kivy/buildozer android debug
```
Note the system dependencies were also sorted alphabetical to ease
maintenance.
Later more dependencies can be added if needed to build a given recipe.
- installs cutting edge version from current source
- removes outdated java version fixes
- migrates to host Python3
- verifies the image builds and run via Travis
- fixes minor linting: "E117 over-indented"
Installs buildozer and dependencies, documents usage.
To build the image you first need to `cd` to where the `Dockerfile` is,
then run:
```sh
docker build --tag=buildozer .
```
Then you can use the container e.g. with:
```sh
docker run --volume "$(pwd)":/home/user/hostcwd buildozer --help
```
The above command mounts host current working directory in container
using `--volume` option.
Also added workaround for #625
