Docker-compose is not available on arm64

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.