Install pg_cron extension on Postgres:13-alpine docker image

Question

I am trying to implement a cron job on my Postgres DB using the pg_cron extension but getting the following error which implies pg_cron extension is not installed on the Postgres:13-alpine image I am using :

 Reason: liquibase.exception.DatabaseException: ERROR: could not open extension control file "/usr/local/share/postgresql/extension/pg_cron.control": No such file or directory [Failed SQL: (0) CREATE EXTENSION pg_cron]

My Dockerfile :

FROM openjdk:11-jre-slim

# Non-root user
USER 1984:1984

COPY --chown=1984:1984 target/*.jar app.jar

CMD ["java","-jar","app.jar"]

My docker-compose file :

version: '3'
services:

  postgres:
    image: postgres:13-alpine
    ports:
      - "5432:5432"
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: pwd
      POSTGRES_DB: db
    volumes:
      - postgresdata:/var/lib/postgresql/data

volumes:
    postgresdata:

AFAIU, I should add the following settings to postgresql.conf

shared_preload_libraries = 'pg_cron' cron.database_name = 'postgres'

but since I am using a packaged Postgres Docker image, I am a bit confused and can't really see how this can be done.

Would you please have any pointers as to how add this pg_cron extension to the alpine image and which changes I have to make to my Dockerfile and docker-compose ?

Thank you.