[frontend] Set environment variable in docker

The docker:test:lint rake task was failing because of git-cop
encoding issues.
It turned out that different encoding in a git message could cause
git-cop to fail. Until this is fixed we need to work around this by
setting the encoding in the docker container.

1: from /usr/lib64/ruby/gems/2.5.0/gems/git-cop-1.7.1/lib/git/cop/
     commits/saved.rb:71:in `method_missing'
/usr/lib64/ruby/gems/2.5.0/gems/git-cop-1.7.1/lib/git/cop/commits/
  saved.rb:71:in `[]': invalid byte sequence in US-ASCII (ArgumentError)