Update Celery Dockerfile to use 'latest' tag for backend image

[RishabhJain2018]

• Update Celery Dockerfile to use 'latest' tag for backend image

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.94%. Comparing base (96968d6) to head (f4e2622).
⚠️ Report is 1236 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4775      +/-   ##
==========================================
+ Coverage   72.93%   76.94%   +4.00%     
==========================================
  Files          83       21      -62     
  Lines        5368     3608    -1760     
==========================================
- Hits         3915     2776    -1139     
+ Misses       1453      832     -621     

see 75 files with indirect coverage changes
see 75 files with indirect coverage changes

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