Parallelize docker image saves in pg_save_image

[Copilot]

The pg_save_image target saved 5 Docker images sequentially via docker save | gzip, taking ~94s total — with wal-g/golang alone consuming ~44s.

Change

Run all docker save operations as concurrent background jobs within a single shell command:

# Before: sequential (~94s)
docker save ${IMAGE} | gzip -c > ${CACHE_FILE_DOCKER_PREFIX}
docker save wal-g/ubuntu:18.04 | gzip -c > ${CACHE_FILE_UBUNTU_18_04}
...

# After: parallel (~44s, bounded by slowest image)
docker save ${IMAGE} | gzip -c > ${CACHE_FILE_DOCKER_PREFIX} & \
    docker save wal-g/ubuntu:18.04 | gzip -c > ${CACHE_FILE_UBUNTU_18_04} & \
    ...
    wait

Expected savings: ~50s per run on the "Build images" job.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[FEATURE] speedup image build</issue_title>
<issue_description>### What feature would you like?

we need to speedup this job https://github.com/chipitsine/wal-g/actions/runs/22643325878/job/65624841607

Why is this feature useful?

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [FEATURE] speedup image build #50

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