dokku/plugins/postgres/docs/create.md

Official Postgres "$DOCKER_BIN" image ls does not include postgis extension (amongst others). The following example creates a new postgres service using postgis/postgis:13-3.1 image, which includes the postgis extension.

# use the appropriate image-version for your use-case
dokku postgres:create postgis-database --image "postgis/postgis" --image-version "13-3.1"

To use pgvector instead, run the following:

# use the appropriate image-version for your use-case
dokku postgres:create pgvector-database --image "pgvector/pgvector" --image-version "pg17"