MSR cache configuration

MSR caches are based on Docker Registry, and use the same configuration file format. The MSR cache extends the Docker Registry configuration file format, though, introducing a new middleware called downstream with three configuration options: blobttl, upstreams, and cas:

middleware:
  registry:
      - name: downstream
        options:
          blobttl: 24h
          upstreams:
            - <Externally-reachable address for upstream registry or content cache in format scheme://host:port>
          cas:
            - <Absolute path to next-hop upstream registry or content cache
              CA certificate in the container's filesystem>

The following table offers detail specific to MSR caches for each parameter:

Parameter	Required	Description
blobttl	no	The TTL (Time to Live) value for blobs in the cache, offered as a positive integer and suffix denoting a unit of time. Valid values: - ns (nanoseconds) - us (microseconds) - ms (milliseconds) - s (seconds) - m (minutes) - h (hours) Note: If the suffix is omitted, the system interprets the value as nanoseconds. If blobttl is configured, storage.delete.enabled must be set to true.
cas	no	An optional list of absolute paths to PEM-encoded CA certificates of upstream registries or content caches.
upstreams	yes	A list of externally-reachable addresses for upstream registries of content caches. If you specify more than one host, it will pull from registries in a round-robin fashion.