Dockerfile: remove pg_isready

[pjonsson]

After adding health checks to docker compose,
this command is no longer used anywhere else.

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📚 Documentation preview 📚: https://datacube-ows--1057.org.readthedocs.build/en/1057/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.64%. Comparing base (8466b07) to head (73732af).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1057   +/-   ##
=======================================
  Coverage   93.64%   93.64%           
=======================================
  Files          43       43           
  Lines        6510     6510           
=======================================
  Hits         6096     6096           
  Misses        414      414           

[pjonsson]

Edit: #1059 removes the copying along with not installing the test dependencies in the prod image, so that supersedes this PR. I'm most likely busy with other things for the next couple of weeks, so if that PR isn't merged or needs further discussion before merging, I think this PR should be merged in the meantime.

The copy statements seem to originate from #743, and that references GDAL as inspiration. I use GDAL 3.4 which is from late 2021 and that does copy, but it's the extra libraries that were built in the builder image that are copied from the builder, not the content of the deb packages.

I'm pretty sure that dodging the package system confuses Trivy since it can't see that the binary comes from a security patched Debian/Ubuntu package, so the image is flagged as containing more security vulnerabilities than it actually does.

The point of this PR was just to remove the now unused binary, I'll fix the rest of the copying in a separate PR. I suspect that will be a bit of work and I have quite a few other things on my plate, so don't hold the next release while you wait for that PR, the image on current master is strictly better than it was in the previous release and I think that is a win in itself.

[SpacemanPaul]

I wouldn't overthink anything done in #743