bump Dockerfile to ruby 2.6

[deargle]

on 2.5, gem install bundler was throwing the following error, so docker build . wasn't working:

+ gem install bundler
ERROR:  Error installing bundler:
	The last version of bundler (>= 0) to support your Ruby & RubyGems was 2.3.26. Try installing it with `gem install bundler -v 2.3.26`
	bundler requires Ruby version >= 2.6.0. The current ruby version is 2.5.9.229.