Simple mechanism for installing extension packages into docker container

[msullivan]

EdgeDB 6.0 supports extension packages (namely postgis), and the cli can download and install them.

We probably want some easy way to cause an extension package to be installed in the container.

See geldata/gel#7770

[bostelk]

I'm running into this issue right now. If you need a vote of confidence that it's worth implementing. That and I assume schema migrations I suppose. It's not clear how to incorporate these two concepts. I saw the Docker COPY command to migrations, so now I have:

Applying m1sg5xzfaderbvmluplpcbkoo3szgpplbkp2ciwzof2cebaxwtrtgq (00001-m1sg5xz.edgeql)
gel error: SchemaError: cannot create extension 'postgis': extension package 'postgis' version '3.5' does not exist
gel error: error in one of the migrations
ERROR: Migrations failed. Stopping server.

I tried packaging a "01-install-postgis.sh" into the boostrap early directory but the server isn't running, I'm not sure how to run a temp server.

01-install-postgis.sh

#!/usr/bin/env bash
gel instance list
gel extension install postgis

output

run-parts: executing /gel-bootstrap.d/01-install-postgis.sh
No instances found
gel error: Instance name argument is required, use '-I name'
run-parts: /gel-bootstrap.d/01-install-postgis.sh exited with return code 2