I worked around this by installing it manually using
language: minimal
os: linux
dist: bionic
arch: arm64
addons:
apt:
update: true
packages:
- python3-dev
- python3-pip
- python3-setuptools
- python3-wheel
install:
- pip3 install -IU docker-compose
(language: python
wouldn’t work: "language: python" seems to force amd64), but it takes a long time and is pretty inconvenient.