Fix Docker release process for dbt-postgres

[mikealfare]

resolves #9827

Problem

We updated the Docker release process for the new location of dbt-postgres. This broke the process for previous versions of dbt-postgres, which still live in dbt-core's repo.

Solution

• add the subdirectory back to the repo reference, which only works for dbt-postgres<1.8
• handle the docker releases for dbt-postgres>=1.8 in the new dbt-postgres repo

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX
• This PR includes type annotations for new and modified functions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.07%. Comparing base (f683e36) to head (d3c886e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9828      +/-   ##
==========================================
- Coverage   88.10%   88.07%   -0.04%     
==========================================
  Files         178      178              
  Lines       22458    22458              
==========================================
- Hits        19786    19779       -7     
- Misses       2672     2679       +7     

Flag	Coverage Δ
integration	85.43% <ø> (-0.04%)	⬇️
unit	61.78% <ø> (ø)

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