Maintain deterministic order in withCopyFileToContainer

[findepi]

withCopyFileToContainer can be invoked multiple times, including with
one containerPath being prefix of another containerPath.

This change makes the end result deterministic.

Extracted from #2861

[findepi]

is the CI failure related? please help me understand.

[rnorth]

Please could you merge from master? There were issues in the build related to not-well-pinned docker image dependencies, which we've since fixed.

Still, like #2861 I think japicmp will complain about a public API change on generated accessors, because of the field type change.

Is this PR necessary if we go for #2861?

[findepi]

Please could you merge from master? There were issues in the build related to not-well-pinned docker image dependencies, which we've since fixed.

thanks for the tip, rebased

Still, like #2861 I think japicmp will complain about a public API change on generated accessors, because of the field type change.

The field is still declared as Map<MountableFile, String>, just initialized differently.

Is this PR necessary if we go for #2861?

This PR is simple improvement that should cause minimal concerns, yet fixes the indeterminism.
If you do not have strong preferences, I would prefer to work on this first.