Multistage, add pg_jobmon too

[aschereT]

1. Use multistage Dockerfile. This allows the final image to avoid including build dependencies such as GCC
2. Include pg_jobmon, an optional dependency of pg_partman

The size of the final image for PostgreSQL 17 and pg_partman 5 is reduced from 630 MB to 374 MB (arm64).

[Morl99]

Thank you, using Multi Stage is a great idea. And since pg_jobmom is listed as a recommended extension on the pg_partman docs, I see no reason to not include it in this image.

I just have a single request to change all the ARG values so that they only appear once in the Dockerfile.

And please add a line to the Readme that pg_jobmon is included.

[Morl99]

One more thing, we need to have #25 in mind and maybe even fix it before merging this ;-) Alternatively we can change the base branch and pull the code into a staging branch to run the build.

[Morl99]

Since I cannot find an easy fix for #24 and I do not have the time to play around with different permissions/tokens, I will merge this into another branch and the create a separate MR to merge this into main, where the workflows will run.

Thank you @aschereT for the contribution.