Fix Dockerfile

[fixdocker]

Hello, I represent a research group investigating build failures in
Dockerfiles. We executed a Dockerfile in your project and found that
it is not building. We are sending this Pull Request with a suggested
repair. The repair operation is based on the following URL:

• ERROR: Error installing bundler: bundler requires Ruby version >= 2.3.0. rubygems/bundler#6865

Please note that we are using an anonymous GitHub account because of
double-blind restrictions in the submission process of conferences.

[coveralls]

Coverage remained the same at 80.142% when pulling 1fcf19b on fixdocker:master into 99bf10c on holderdeord:master.

[fixdocker]

Hey @jarib , can you check this PR?