docker/keycloak: Fix custom swh theme installation for Keycloak 12

Since Keycloak 12, it is no longer possible to use a custom theme from a directory symbolic link, so make a raw copy of the swh theme in the Keycloak theme folder instead.

For the record, I opened an issue on the subject in Keycloak JIRA a couple of weeks ago and I got the following answer:

Rejecting this as a bug, as we want to make sure only files within the given 
directory can be used for security reasons. We never intended to support symlinks, 
which was a side-effect of a bug/vulnerability that was resolved.

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Migrated from D5119 (view on Phabricator)

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