Multi-arch image vouching fails: failed to load manifest with status 404 Not Found

Nobody has claimed this yet.

I'm trying to vouch for a multi-arch image and getting this response:

```
failed to load manifest with status 404 Not Found:
"{"errors":[{
"code":"MANIFEST_UNKNOWN",
"message":"Manifest with digest 'sha256:XXX' has media type
'application/vnd.docker.distribution.manifest.list.v2+json',
but client accepts 'application/vnd.docker.distribution.manifest.v2+json,application/vnd.docker.distribution.manifest.v1+json,application/vnd.docker.distribution.manifest.v1+prettyjws'."}]}"
```

the request is:

```
http.post(
'https://voucher.acme.com/diy',
headers: {
'Authorization' => "Basic #{basic_authentication}",
'Content-Type' => 'application/json',
},
max_attempts: 4,
body: {
image_url: canonical_image_url,
}.to_json
```

and the manifest is:
```yml
{
"mediaType": "application/vnd.docker.distribution.manifest.list.v2+json",
"schemaVersion": 2,
"manifests": [
{
"mediaType": "application/vnd.docker.distribution.manifest.v2+json",
"digest": "sha256:1512682275109406d73565ec11b8bd3aeaf53bfac83e8e12202a3effd0ae8ea4",
"size": 528,
"platform": {
"architecture": "arm",
"os": "linux",
"variant": "v7"
}
},
{
"mediaType": "application/vnd.docker.distribution.manifest.v2+json",
"digest": "sha256:f587207cab008ab273d77a3b1c9f9c983888fa659a46e1284c1984272e4dc208",
"size": 528,
"platform": {
"architecture": "arm64",
"os": "linux"
}
},
{
"mediaType": "application/vnd.docker.distribution.manifest.v2+json",
"digest": "sha256:fbb75455e35366f89fe310e88420a1a35a15ee54bc86aeef36fd561de68ce5ae",
"size": 528,
"platform": {
"architecture": "amd64",
"os": "linux"
}
}
]
}
```

Start by tracing how the /diy request loads and validates the Docker image manifest, using the reported manifest list media type and 404 response as the reproduction. Check how accepted manifest media types are selected, then verify that the supplied multi-arch image can be vouched successfully without the MANIFEST_UNKNOWN response.

Written by the indexing model from the issue text.