containers: Set Unicode locale in unit-tests container
containers: Set Unicode locale in unit-tests container

We often process non-ASCII strings in our tests and logs.  Set a proper
Unicode locale to avoid Unicode{En,De}codeErrors when running tests
manually in the container.

Closes #9594 
cockpituous/cockpit,andreasn/cockpit,mvollmer/cockpit,mareklibra/cockpit,andreasn/cockpit,cockpituous/cockpit,mareklibra/cockpit,garrett/cockpit,mvollmer/cockpit,mareklibra/cockpit,mareklibra/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,cockpituous/cockpit,andreasn/cockpit,cockpit-project/cockpit,deryni/cockpit,mareklibra/cockpit,moraleslazaro/cockpit,deryni/cockpit,mvollmer/cockpit,cockpit-project/cockpit,mareklibra/cockpit,martinpitt/cockpit,garrett/cockpit,andreasn/cockpit,deryni/cockpit,martinpitt/cockpit,mvollmer/cockpit,andreasn/cockpit,garrett/cockpit,moraleslazaro/cockpit,andreasn/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,martinpitt/cockpit,andreasn/cockpit,deryni/cockpit,deryni/cockpit,garrett/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,cockpituous/cockpit,cockpit-project/cockpit,garrett/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,deryni/cockpit,mareklibra/cockpit,deryni/cockpit,martinpitt/cockpit