Fix mbed-os-env docker digest retrieval

[saheerb]

Adapt changes in skopeo command line to inspect docker image.
Use tagged skopeo docker image to avoid breakage in future.

Summary of changes

mbed-os-env docker images bundles environment to quick start mbed-os development. GithHub Actions in this repository builds and publishes the docker images to GitHub Container Registries (GHCR). In the GitHub Action workflow, skopeo docker image manages the docker images in GHCR. Latest version of skopeo has changed the way some commands are executed. As the GitHub Actions were using latest skopeo images, currently these GitHub Actions are failing. This PR adapted the skopeo command line changes and also attached the skopeo usage to a particular tag version.

This PR also tags the images generated by master branch to 'latest'.

Impact of changes

No

Migration actions required

None

Documentation

None

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[X] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

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[ciarmcom]

@saheerb, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

Pull request has been modified.

[ciarmcom]

@saheerb, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

[0xc0170]

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