Ansible can't import docker even though it's installed - docker

Question

I'm trying to build a server that runs a docker container using ansible, but I'm getting the error Failed to import the required Python library (Docker SDK for Python: docker (Python >= 2.7) or docker-py (Python 2.6)) on ubuntu-xenial's Python /usr/bin/python3.
The target machine is ubuntu xenial, currently in vagrant but I get the same error on an Azure VM. My version of ansible is 2.9.15, the docker version ansible is installing is 20.10.1.
My test inventory file consists of
all:
vars:
ansible_user: vagrant
ansible_become: yes
ansible_python_interpreter: /usr/bin/python3
children:
a_servers:
hosts:
192.168.33.100
b_servers:
hosts:
192.168.33.100
prod:
hosts:
192.168.33.100
The relevant part of my playbook is:
- name: Docker role
include_role:
name: nickjj.docker
vars:
docker__users: ['deploy']
docker_registries:
- #registry_url: "https://index.docker.io/v1/"
username: "{{ docker_user.username }}"
password: "{{ docker_user.password }}"
- name: Log in to docker
docker_login:
username: "{{ docker_user.username }}"
password: "{{ docker_user.password }}"
become: yes
become_user: deploy
When I log in to the vagrant box as user deploy, I can run docker login and docker-compose up -d without any problems. I can also run /usr/bin/python3 and import docker without issue.
There is no other python on the system.
The error comes from the docker_login resource, which can't seem to import docker.
Is a configuration I'm missing, or something else I've overlooked that would cause it to fail? Any help is much appreciated.