Set the locale in the Docker image

[replaceafill]

While preparing a few test transfers with non ascii paths I realized that if a feature file includes non ascii text the make test-at-behave rule of the development environment currently fails like this:

    Given a "standard" transfer type located in "Exception UnicodeEncodeError: 'ascii' codec can't encode characters in position 23-28: ordinal not in range(128)
Traceback (most recent call last):
  File "/usr/local/bin/behave", line 11, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.6/dist-packages/behave/__main__.py", line 183, in main
    return run_behave(config)
  File "/usr/local/lib/python3.6/dist-packages/behave/__main__.py", line 127, in run_behave
    failed = runner.run()
  File "/usr/local/lib/python3.6/dist-packages/behave/runner.py", line 804, in run
    return self.run_with_paths()
  File "/usr/local/lib/python3.6/dist-packages/behave/runner.py", line 824, in run_with_paths
    return self.run_model()
  File "/usr/local/lib/python3.6/dist-packages/behave/runner.py", line 626, in run_model
    failed = feature.run(self)
  File "/usr/local/lib/python3.6/dist-packages/behave/model.py", line 321, in run
    failed = scenario.run(runner)
  File "/usr/local/lib/python3.6/dist-packages/behave/model.py", line 1114, in run
    failed = scenario.run(runner)
  File "/usr/local/lib/python3.6/dist-packages/behave/model.py", line 711, in run
    if not step.run(runner):
  File "/usr/local/lib/python3.6/dist-packages/behave/model.py", line 1311, in run
    formatter.match(match)
  File "/usr/local/lib/python3.6/dist-packages/behave/formatter/pretty.py", line 132, in match
    self._match.location, self.monochrome)
  File "/usr/local/lib/python3.6/dist-packages/behave/formatter/pretty.py", line 301, in print_step
    self.stream.write(arg_format.text(arg.original))
UnicodeEncodeError: 'ascii' codec can't encode characters in position 23-28: ordinal not in range(128)

This fixes it by installing the locales package and generating the en_US.UTF-8 locale in the Docker image which is the same approach we use in the hack/Dockerfile.

Connected to archivematica/Issues#1317