Add Slack notification for image build workflow failure

[NymanRobin]

Adds Slack notification for EST internal teams slack channel for image build workflow failures. This is so a wider audience can more quickly detect failures and avoid scenarios such as #19 where the build was broken for months before anyone noticed.

Also fixed the indent in the file to be consistent with the other repositories in metal3 org

Sadly, this is currently only EST internal hopefully if this works out well we can find a solution were all interested can partake

Fixes: #20

[metal3-io-bot]

@tuminoid: adding LGTM is restricted to approvers and reviewers in OWNERS files.

In response to [this](https://github.com/metal3-io/ironic-client/pull/23#pullrequestreview-2084469462): >/lgtm Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

[NymanRobin]

/cc @dtantsur @elfosardo Could you please have a look at this? It helps us quite a lot in detecting errors quickly

[elfosardo]

/approve

[metal3-io-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: elfosardo

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[dtantsur]

/lgtm