docker: Libervia revision can now be specified: Using the `DOCKER_LIBERVIA_REV` environment variable, backend (and related repos) revision can now be specified. By default (variable empty or not set), the current `tip` version is used, and `dev` is used for tagging. If the variable is set, the revision will be used and used for tagging the image.

You'll find install instructions in documentation (see doc/installation.rst).

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