Automated toolchain install.

[jklmnn]

The current hack of piping the output of yes into doinstall isn't the way to go. Provide a better way to automatically install the GNAT and SPARK toolchain.

[reznikmm]

Gnat community 2018 has completely different installer. AdaCore provides installation script here:

https://github.com/AdaCore/gnat_community_install_script

[jklmnn]

I already forgot about this issue. For another project I built a fork of your GNAT Docker container that also installs SPARK 2018. I should update this repo to use this.

[jklmnn]

I have adapted the gnat community install script for the current version.