fix: docker pull fallback on linux amd64 for apple silicon

[sevbch]

Context

A user reported the following issue on Apple Silicon:

ggshield secret scan docker bimapangestu/pertamina-login:latest
Saving docker image...
need to download image first
latest: Pulling from bimapangestu/pertamina-login
no matching manifest for linux/arm64/v8 in the manifest list entries

Error: Image "bimapangestu/pertamina-login:latest" not found

After pulling the image separately (using docker pull bimapangestu/pertamina-login:latest or docker pull --platform linux/amd64 bimapangestu/pertamina-login:latest), it ended working.

What has been done

When docker pull command fails, retry pulling specifically the linux/amd64 image.

Validation

Use case on Apple Silicon:

• Make sure not to have locally pulled bimapangestu/pertamina-login:latest image (docker rmi -f bimapangestu/pertamina-login:latest)
• run ggshield secret scan docker bimapangestu/pertamina-login:latest on main: command fails
• On this branch: command works

Extra comment

I first added the fallback only for Apple Silicon OS and was advised to make it available for every kind of platform. I'm not opinionated on the topic.

PR check list

• As much as possible, the changes include tests (unit and/or functional)
• If the changes affect the end user (new feature, behavior change, bug fix) then the PR has a changelog entry (see doc/dev/getting-started.md). If the changes do not affect the end user, then the skip-changelog label has been added to the PR.

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.88%. Comparing base (bc4777c) to head (c8835dd).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
ggshield/verticals/secret/docker.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1104      +/-   ##
==========================================
- Coverage   91.89%   91.88%   -0.01%     
==========================================
  Files         144      144              
  Lines        6094     6102       +8     
==========================================
+ Hits         5600     5607       +7     
- Misses        494      495       +1     

Flag	Coverage Δ
unittests	91.88% <90.00%> (-0.01%)	⬇️

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.