Non functional Docker build

[wynro]

Currently the make build command produces an error, because the last version of pry-byebug requires ruby >= 2.2.0, and the default release with Debian Jessie is 2.1, so the build produces an error when trying to install the gem.

[wynro]

For just building the program (although I guess is not applicable to debugging), just removing the pry-byebug produces a functional build.