By default, composer pulls from github using the github API, which is rate limited.
Composer is called every time docker-compose brings the container up, so we reach the limit rather quickly with enough restarts. The container will fail to start because the github API starts requiring authentication.
To fix, need to tell composer not to use the API in composer.json:
{
"type": "vcs",
"url": "https://github.com/IMSGlobal/lti-1-3-php-library",
"no-api": true
}