Improve formatted output of oras manifest fetch

[FeynmanZhou]

What happened in your environment?

I want to use oras command to only see the layers in JSON format, but it returns the result as follows which looks not friendly to users:

$ oras manifest fetch ghcr.io/deislabs/ratify/notary-image:signed --format {{.Content.layers}}
[map[digest:sha256:2408cc74d12b6cd092bb8b516ba7d5e290f485d3eb9672efc00f0583730179e8 mediaType:application/vnd.docker.image.rootfs.diff.tar.gzip size:2.798889e+06] map[digest:sha256:89b865d5710c99cddea45ece6dec6e01a7fea6fc62ce28f667fb7bf26b124b82 mediaType:application/vnd.docker.image.rootfs.diff.tar.gzip size:165] map[digest:sha256:2558cbd97e0483e3bd0c66a3f03f471949c2a6c2fcb5d48a8ca577f22f1653fa mediaType:application/vnd.docker.image.rootfs.diff.tar.gzip size:110]]

If I use --pretty to prettify the JSON output, it doesn't work:

oras manifest fetch ghcr.io/deislabs/ratify/notary-image:signed --format {{.Content.layers}} --pretty
[map[digest:sha256:2408cc74d12b6cd092bb8b516ba7d5e290f485d3eb9672efc00f0583730179e8 mediaType:application/vnd.docker.image.rootfs.diff.tar.gzip size:2.798889e+06] map[digest:sha256:89b865d5710c99cddea45ece6dec6e01a7fea6fc62ce28f667fb7bf26b124b82 mediaType:application/vnd.docker.image.rootfs.diff.tar.gzip size:165] map[digest:sha256:2558cbd97e0483e3bd0c66a3f03f471949c2a6c2fcb5d48a8ca577f22f1653fa mediaType:application/vnd.docker.image.rootfs.diff.tar.gzip 

What did you expect to happen?

Show the layers in prettified JSON output:

$ oras manifest fetch ghcr.io/deislabs/ratify/notary-image:signed --format {{.Content.layers}} --pretty
[
      {
        "digest": "sha256:2408cc74d12b6cd092bb8b516ba7d5e290f485d3eb9672efc00f0583730179e8",
        "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
        "size": 2798889
      },
      {
        "digest": "sha256:89b865d5710c99cddea45ece6dec6e01a7fea6fc62ce28f667fb7bf26b124b82",
        "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
        "size": 165
      },
      {
        "digest": "sha256:2558cbd97e0483e3bd0c66a3f03f471949c2a6c2fcb5d48a8ca577f22f1653fa",
        "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
        "size": 110
      }
]

How can we reproduce it?

See above

What is the version of your ORAS CLI?

Download it with the following steps:

VERSION="1.2.0-manifest-fetch"
curl -LO "
[https://github.com/qweeah/oras/releases/download/v${VERSION}/oras_${VERSION}_linux_amd64.tar.gz"](https://github.com/qweeah/oras/releases/download/v$%7BVERSION%7D/oras_$%7BVERSION%7D_linux_amd64.tar.gz%22)
mkdir -p oras-install/
tar -zxf oras_${VERSION}_*.tar.gz -C oras-install/
rm -rf oras_${VERSION}_*.tar.gz

What is your OS environment?

Ubuntu 22.06

Are you willing to submit PRs to fix it?

• [ ] Yes, I am willing to fix it.

[qweeah]

This is expected, you need to use default spring function to convert it to json, e.g.

> oras manifest fetch ghcr.io/deislabs/ratify/notary-image:signed --format "{{toPrettyJson .content.layers}}"