Make docker init script respect COCKROACH_DATABASE when processing SQL files

[mecab]

This PR makes the docker init script run the SQL files in docker-entrypoint-initdb.d against the database specified by COCKROACH_DATABASE.

The behaviour is aligned to Postgres's init script, and I believe it is beneficial for the most of use cases.

Should close #137735

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[cockroach-teamcity]

All committers have signed the CLA.

[cockroach-teamcity]

This change is 

[rafiss]

thanks for your contribution! this looks good to me. i will merge it once the tests complete.

[rafiss]

bors r+

[craig]

Build succeeded:

• check_generated_code
• local_roachtest
• unit_tests
• acceptance
• linux_amd64_fips_build
• lint
• docker_image_amd64
• local_roachtest_fips
• linux_amd64_build
• examples_orms

[mecab]

Thanks @rafiss and the team for handling and merging it!