build: support `IMAGE_TAG` build variable

[jacwil]

Makes it easier to build selfservice ui locally

Checklist

• [x] I have read the contributing guidelines and signed the CLA.
• [ ] I have referenced an issue containing the design document if my change introduces a new feature.
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• [x] I confirm that this pull request does not address a security vulnerability. If this pull request addresses a security vulnerability, I confirm that I got approval (please contact security@ory.sh) from the maintainers to push the changes.
• [ ] I have added tests that prove my fix is effective or that my feature works.
• [ ] I have added the necessary documentation within the code base (if appropriate).

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[jacwil]

Abandoning. Realized that my commit did not have my verified github email address, which is causing CLA check to fail.