admission: AlwaysPullImages: support ephemeral containers

[xdavidwu]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Add ephemeralcontainers subresource support to AlwaysPullImages. Before this, ephemeralcontainers subresource request was not handled and it was possible to add ephemeral containers with imagePullPolicy other than Always.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Fixed a bug in AlwaysPullImage admission plugin where ephemeralcontainers subresource of Pod was not handled.

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @xdavidwu. Thanks for your PR.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: xdavidwu
Once this PR has been reviewed and has the lgtm label, please assign deads2k for approval. For more information see the Kubernetes Code Review Process.

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Needs approval from an approver in each of these files:

• plugin/pkg/admission/OWNERS

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[pacoxu]

/cc @deads2k @ncdc

and @verb for ephemeral containers