Docker and podman can be configured to use mutual TLS authentication
to the registry by dropping files into system-wide and user
directories. Implement this in a largely compatible way.
(Because of the limitations of our underlying libraries, we
can't support multiple certificates within the same host config,
but I don't expect anybody actually needs that.)
The certs.d handling is extended so that certificates are separately
looked up when downloading the look-aside index. This is mostly
to simplify our tests, so we can use one web server for both -
in actual operation, we expect the indexes to be unauthenticated.
Also for testing purposes, FLATPAK_CONTAINER_CERTS_D is supported
to override the standard search path.
Co-authored-by: Sebastian Wick <sebastian.wick@redhat.com>
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