Is there any way to have the docker instance set up with composer installed? #5945

act does not have a special “install Composer into every runner” setting. The reproducible choices are the same ones you would use on GitHub Actions: select a runner image that already contains what you need, add a setup action/step, or build your own runner image.

For a workflow-level solution, use Composer's setup action before composer install:

jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: shivammathur/setup-php@v2
        with:
          php-version: '8.3'
          tools: composer:v2
      - run: composer install --no-interaction --prefer-dist

If this works on GitHub but fails under act, the runner image is probably too small. act's micro image is intentionally incomplete. Map the workflow label to a fuller image:

act -P ubuntu-latest=catthehacker/ubuntu:act-latest

For fully offline/repeatable local runs, build a custom runner image with PHP/Composer already installed and map ubuntu-latest to it. Use --pull=false if you want act to keep the local image instead of checking a registry.

Do not add Composer to act's ephemeral container manually after it starts; that state disappears with the container and makes local behavior diverge from the workflow. Putting setup in YAML also keeps GitHub-hosted and local runs aligned.

If the setup action or fuller runner image resolves the workflow, please mark this as the accepted answer.