It could be really cool if I can give a custom docker image of the database.

I would also love to know what the plan is for an upgrade to PG 14 and PostGIS 3+ as the current PG 13 and PostGIS 2.5 versions lack a lot of performance improvements that I need for geospatial visualisations.
I would say this is the number 1 reason holding me back from really investing into the Railway ecosystem for my geospatial projects. While I can get by with PG13, it'll make a huge difference for me to be able to utilise the latest features in PG14+ and PostGIS 3+.
I looked into this a bit and it seems the Timescale Docker image has removed support for PostGIS [1] so if you do upgrade to PG14 please can I request it includes PostGIS. The Timescale folk suggest their other Docker image which does have support for both PG14 and PostGIS 3+ [2].
Alternatively there are various other Docker images for PG and PostGIS, so perhaps one option is to support a couple of well-supported Docker images so people can choose from a small selection when provisioning a DB, rather than being limited to only the Timescale one (eg. I don't need Timescale myself).
I also asked about this on Twitter and on Discord [3] but haven't had a response yet (not a problem!), just saying that here so you know it's me asking in various places ;)
[1] https://github.com/timescale/timescaledb-docker/issues/182#issuecomment-1107932273
[2] https://github.com/timescale/timescaledb-docker-ha/
[3] https://discord.com/channels/713503345364697088/1031673325769998377/1031673325769998377