Add Postgis extension to the docker image available extensions.

[muntdan]

Postgis extension is one of the most popular extension/motive to use postgres.
and "Postgis is already available in Hydra's cloud product." according to @wuputah

[saputradharma]

Hi @wuputah, with this PR merged, adding postgis extension to the docker image can be done with pgxman, right?

[wuputah]

pgxman is in early development so... stay tuned! The intention will be that you can have pgxman install extensions without having to modify the image, e.g. just by setting an environment variable. We welcome input and contributions over on pgxman/pgxman or in Discord.

[saputradharma]

Awesome! I realized that adding the new extension in the Docker build step makes longer build time. Excited to see more about pgxman.