Locale testing

#69 (with the advent of Boost.Charconv) solves a problem of locale-dependency. We should have tests to verify that we don't fail parsing regardless of the locale used.

Note that Docker images need to be updated to support the locale under test.

Start by reading issue #69 and the existing locale-sensitive parsing tests, then inspect the Docker image definitions used by the test environment. Add coverage that exercises parsing under the locale described in the issue and update the images to support it. Done means the tests pass consistently regardless of the locale used.

Written by the indexing model from the issue text.