Change license from MIT to GPLv3

[karmacoma]

As of lisk-docker version 1.3.4, we are changing the licensing scheme from MIT to to GPLv3.

The main reason for this change is to provide protection against the future use of lisk-docker in a proprietary form, while at the same time maintaining all of the freedoms to use, change or share the project which are currently in-place.

For more information on GPLv3 please read: https://www.gnu.org/licenses/quick-guide-gplv3.html

Tasks:

• Change license footer, with amendment to reflect change of license
• Add license badge to README.md
• Include license file within project