github action fix for uppercase repository owner

[saheerb]

Summary of changes

docker repository can only be lower case. Hence, pushing to docker repository with name "ARMMbed" fails. This PR fixes the bug by converting the docker repo name to lower case before pushing.

Impact of changes

Migration actions required

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

@jamesbeyond

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[0xc0170]

CI started