Remove duplicate GHA step to build image

[theseanything]

The old build step was accidentally re-introduced to the docker_build GitHub Action workflow in commit ba01fe672688b6303da1d5008a6ea7471d664dbb

This meant we were building images twice as part of CI.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

🎉 A review copy of this PR has been deployed! It is made of up two components

1. A review copy of forms-runner
2. A production copy of forms-admin

Important

Not all of the functionality of forms-runner is present in review apps.
Functionality such as sending emails, file upload, and S3 submission types are
deliberately disabled for the sake of simplifying review apps.

You should use the full dev environment to test the functionality which is disabled here.

It may take 5 minutes or so for the application to be fully deployed and working. If it still isn't ready
after 5 minutes, there may be something wrong with the ECS task. You will need to go to the integration AWS account
to debug, or otherwise ask an infrastructure person.

For the sign in details and more information, see the review apps wiki page.