m/node/core/update: add support for OCI index

The updater now also accepts OCI indexes, and will select the image with
the matching architecture from the index.

This is tested by making on of the update steps a multiarch index.
The testos_multiarch_* targets are tagged "manual" such that test //...
does not build it for all variants.

Change-Id: I2682beb1adf61de0e86c53371c63c4fd9afecf08
Reviewed-on: https://review.monogon.dev/c/monogon/+/4478
Tested-by: Jenkins CI
Reviewed-by: Tim Windelschmidt <tim@monogon.tech>
diff --git a/metropolis/node/core/update/BUILD.bazel b/metropolis/node/core/update/BUILD.bazel
index ed98d21..ff2faea 100644
--- a/metropolis/node/core/update/BUILD.bazel
+++ b/metropolis/node/core/update/BUILD.bazel
@@ -22,6 +22,7 @@
         "//osbase/oci/osimage",
         "//osbase/oci/registry",
         "@com_github_cenkalti_backoff_v4//:backoff",
+        "@com_github_opencontainers_image_spec//specs-go/v1:specs-go",
         "@org_golang_google_grpc//codes",
         "@org_golang_google_grpc//status",
         "@org_golang_google_protobuf//proto",