feat: oci_pull supports loading Helm charts

[sobotklp]

This MR shows an example of loading a Helm chart using oci_pull from a repository (for example, ECR)

For example, in WORKSPACE:

oci_pull(
    name = "dependency-helm-chart",
    digest = "sha256:865647e7904a1e4b4e130b423d20b34dcc89a5a37b7bc82e30ff100c30cfb86f",
    registry = "111222333444.dkr.ecr.us-east-2.amazonaws.com",
    repository = "helm/dependency-charts",
)

This is related to issue #282

[alexeagle]

Thanks for the PR!!

Could you also try updating a documentation spot on oci_pull so it's easier for users to discover this ability?

[alexeagle]

Not going to merge this yet, as expanding the mime types is just the tip of an iceberg where we will get a larger scope and need to make a conscious choice about that.

[sobotklp]

@alexeagle no worries. I will spend some time adding more documentation and test cases as well.

I'm also having trouble coming up with a general use case for someone (besides me) who wants to use this functionality. Downloading the OCI helm chart is easy enough, but what to do with it afterwards? Any guidance you could give here would be appreciated.

[thesayyn]

That's the rest of the iceberg, aka OCI artifacts, which we are not yet ready to support. Neither rules_oci nor crane is able to do anything with artifacts yet.

On top of my head, I can think of using parts of a pulled helm chart to create a new one and push it with oci_push.

Currently, rules_oci only conforms to the non-artifact part of https://github.com/opencontainers/image-spec, but we are considering adding full support for artifacts in the future.

[anguslees]

Downloading the OCI helm chart is easy enough, but what to do with it afterwards?

I want this, fwiw! I have my own bazel code that can do helm template $chart.tgz, but I would like to use rules_oci to actually fetch the chart file from an OCI repo.

[thesayyn]

I don't think we can accept this now, as the whole story for OCI artifacts is missing here. We need to work on #282 first before we can add this.