Docker gives you the freedom of IRIS version during development: you can switch to ANY IRIS version just by changing one line code in a Dockerfile. E.g. here the lowest before FROM $IMAGE is the one that will be used to work with your solution:

ARG IMAGE=intersystemsdc/irishealth-community:2020.3.0.200.0-zpm

ARG IMAGE=intersystemsdc/iris-community:2020.4.0.547.0-zpm

ARG IMAGE=containers.intersystems.com/intersystems/iris:2021.1.0.215.0

ARG IMAGE=intersystemsdc/iris-community

ARG IMAGE=intersystemsdc/irishealth-community

FROM $IMAGE