The DynamicUser mechanism does not allow share the bridge
registration.yaml file with matrix-synapse in any easy way:
1. the mautrix-whatsapp group under which the service runs is not in
/etc/group, so you can't add matrix-synapse to this.
This makes the group read bit on the file useless as the group is
effectively always empty.
2. It's not possible to use ACLs or copy the file during preStart or
postStart because the commands are executed with low priviledges.
By using a normal (static) user/group the secret can be shared with
synapse as follows:
services.matrix_synapse.settings.app_service_config_files =
[ "/var/lib/mautrix-whatsapp/whatsapp-registration.yaml" ];
users.users.matrix-synapse.extraGroups = [ "mautrix-whatsapp" ];
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