[HHH-18507] allow overwriting the default db image with environment variable in docker_db.sh

[simkam]

…ariable in docker_db.sh

https://hibernate.atlassian.net/browse/HHH-18507

If approved, I will make similar changes to the 6.6 (in WF.next), 6.4 (in WF currently), and 6.2 (in EAP) branches.

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[dreab8]

Thanks @simkam for the PR and @beikov for the review.