Fix checked out branch for PR docker image build workflow

[tsotnet]

Signed-off-by: Tsotne Tabidze tsotne@tecton.ai

What this PR does / why we need it: Looks like by default docker image build & push step checks out master branch, which is not what we want. Instead, we want to check out the PR branch and build the docker image from that codebase.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

NONE

[codecov-commenter]

Codecov Report

Merging #2018 (6c66878) into master (f21d27b) will decrease coverage by 22.87%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #2018       +/-   ##
===========================================
- Coverage   83.22%   60.34%   -22.88%     
===========================================
  Files         103      103               
  Lines        8421     8421               
===========================================
- Hits         7008     5082     -1926     
- Misses       1413     3339     +1926     

Flag	Coverage Δ
integrationtests	?
unittests	60.34% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
.../integration/online_store/test_universal_online.py	14.06% <0.00%> (-84.90%)	⬇️
.../integration/online_store/test_online_retrieval.py	17.39% <0.00%> (-82.61%)	⬇️
...fline_store/test_universal_historical_retrieval.py	19.29% <0.00%> (-80.12%)	⬇️
sdk/python/tests/utils/online_read_write_test.py	20.68% <0.00%> (-79.32%)	⬇️
...gration/registration/test_feature_service_apply.py	31.25% <0.00%> (-68.75%)	⬇️
sdk/python/feast/infra/online_stores/redis.py	26.97% <0.00%> (-67.11%)	⬇️
sdk/python/tests/data/data_creator.py	34.78% <0.00%> (-65.22%)	⬇️
sdk/python/tests/integration/e2e/test_usage_e2e.py	35.00% <0.00%> (-65.00%)	⬇️
...s/integration/registration/test_universal_types.py	36.73% <0.00%> (-63.27%)	⬇️
sdk/python/feast/infra/online_stores/datastore.py	32.28% <0.00%> (-61.42%)	⬇️
... and 49 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f21d27b...6c66878. Read the comment docs.

[felixwang9817]

/lgtm

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: felixwang9817, tsotnet

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [felixwang9817,tsotnet]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[felixwang9817]

/kind housekeeping