Update docker-build.yml to remove workflow_dispatch

[schurpl]

Removed manual trigger for Docker build workflow.

Description of changes

Please provide a short description of the change or if the change is sufficiently described in the GitHub issue, provide a link to the issue.

How has this been tested?

Please describe the test strategy you followed. If not tested, please explain why.

• automated unit test
• automated integration test
• manual, exploratory test

In case of manual test, please document the test well including a set of user instructions and prerequisites. Each including an action, it's result, and where appropriate a screenshot.

Checklist before requesting a review

• My code is easy to understand
• My code follows the coding guidelines of this project
• My code fulfills all acceptance criteria
• The test coverage (line and branch) is reasonably high, especially on Service classes and other classes with complex logic
• I have made corresponding changes to the documentation or
  the wiki/adr
• I made no breaking changes in the database schema or if so, I will perform a database migration

Checklist for reviewer

• The code is easy to understand
• The code follows
  the coding guidelines of
  this project
• The code is tested or if not, the reason is documented or discussed
• The added and existing tests reasonably cover the code change
• The code has no breaking changes in the database schema or if so, the assignee is aware of it and
  will perform a database migration