Add syntax annotation to Dockerfile

[kovayur]

Description

This small change magically makes BuildKit cache mounts work properly on microshift.

Reference: docker/buildx#1283 (comment)

Checklist (Definition of Done)

• Unit and integration tests added
• Added test description under Test manual
• Documentation added if necessary (i.e. changes to dev setup, test execution, ...)
• CI and all relevant tests are passing
• Add the ticket number to the PR title if available, i.e. ROX-12345: ...
• Discussed security and business related topics privately. Will move any security and business related topics that arise to private communication channel.
• Add secret to app-interface Vault or Secrets Manager if necessary
• RDS changes were e2e tested manually
• Check AWS limits are reasonable for changes provisioning new resources
• (If applicable) Changes to the dp-terraform Helm values have been reflected in the addon on integration environment

Test manual

TODO: Add manual testing efforts

# To run tests locally run:
make db/teardown db/setup db/migrate
make ocm/setup
make verify lint binary test test/integration

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kovayur, vladbologa

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