actions: Fix docker build workflow_dispatch

[anomiex]

Proposed changes

When this was added in #46895, it didn't add handling of the event.

Related product discussion/links

p1777316840109259/1777316810.846289-slack-C034JEXD1RD

Does this pull request change what data or activity we track or use?

No

Testing instructions

• You could try dispatching the workflow with this branch; it should fail.
• Merge, then try it with trunk. It should work.

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[tbradsha]

Looks good:
https://github.com/Automattic/jetpack/actions/runs/25014744698/job/73259496807