This is likely an issue with your local environment, It builds fine for us https://travis-ci.org/docker-library/postgres/builds/452492635

This is the only relevant search result for your specific error https://stackoverflow.com/questions/52900636/gpg-command-works-in-shell-but-not-in-go-exec-command
It's possibly related to docker-library/official-images#4252

I recommend using the images we officially build/publish; for further assistance debugging your local environment the best I can recommend is trying the Docker Community Forums, the Docker Community Slack, or Stack Overflow.

So this is an issue with a recent gpg update and we are encountering it, we filed a bug report: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=913614#27

The bug appears to be due to the fix for https://bugs.debian.org/906545
-- where we applied cleaning to keys being imported.

That testing didn't try out all the variants without having access to a tty.

And we've opened a PR on it #527

From the comment in https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=913614#27

PS i do encourage everyone who is automating the use of gpg to use
--batch everywhere, as this forces GnuPG into a mode that is expected to
be used for automation (its "API", for lack of a better term, as opposed
to its "UI", which is its normal non-batch mode). And, FWIW, i agree
with Tianon that GnuPG should simply assume --batch if no tty exists,
but that's not the kind of change i can fit into debian stable, i think.