🌱 Add custom ref input to container build workflow

[mquhuy]

This PR adds a custom inputs, which can be used to pass a custom git ref to the container build workflow. ref_name and sha references will be updated accordingly.
In case the input is not available, the workflow still uses github.ref github.ref_name and github.sha, which is the current behavior.

[tuminoid]

This is needed to enable container image builds from tags via automated release workflow.

Second step is amending metal3-io/ip-address-manager#775 to pass these inputs, right?

[mquhuy]

/cc @lentzi90 @Rozzii @smoshiur1237

[tuminoid]

Minor nits about bash snippets. Let's start formatting those properly, makes life easier.

Otherwise, looks ok.

[Rozzii]

/lgtm

[tuminoid]

/approve

[metal3-io-bot]

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[tuminoid]

/override metal3-ubuntu-e2e-integration-test-main

[metal3-io-bot]

@tuminoid: Overrode contexts on behalf of tuminoid: metal3-ubuntu-e2e-integration-test-main

In response to this:

/override metal3-ubuntu-e2e-integration-test-main

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