Fix docker_build condition to only run on prs

[imnasnainaec]

The syntax was wrong so this line was ignored. The result is that the docker_build jobs run when prs are merged into master which is redundant with the builds for qa deployment.

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This change is 

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.40%. Comparing base (a971f34) to head (bfc3451).
⚠️ Report is 33 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3985   +/-   ##
=======================================
  Coverage   74.40%   74.40%           
=======================================
  Files         291      291           
  Lines       10790    10790           
  Branches     1351     1351           
=======================================
  Hits         8028     8028           
  Misses       2373     2373           
  Partials      389      389           

Flag	Coverage Δ
backend	85.45% <ø> (ø)
frontend	65.86% <ø> (ø)

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[jasonleenaylor]

@jasonleenaylor reviewed 5 of 5 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @imnasnainaec)