Can't generate app key as docker closes cachet container due to missing app key

```
cachet_1 | ERROR: Please set the 'APP_KEY=base64:0jboLlfNau8ejNCizAXBKIa1mHPCRTPpfC70gzFO3sg=' environment variable at runtime or in docker-compose.yml and re-launch
cachet-docker_cachet_1 exited with code 0

```
Environment:
Amazon Linux AMI release 2018.03
Docker version 19.03.6-ce, build 369ce74
docker-compose version 1.25.5, build 8a1c60f6
git version 2.14.5
```
sudo docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
cachet-docker_cachet latest e33e727f75c6 4 minutes ago 376MB
postgres 12-alpine ae192c4d3ada 2 weeks ago 152MB
nginx 1.17.8-alpine 48c8a7c47625 3 months ago 21.8MB
```

Steps to reproduce:
Launch AWS free instance with ami-097834fcb3081f51a
```
yum install git docker -y
git clone https://github.com/cachethq/Docker.git cachet-docker
cd cachet-docker/
sudo curl -L "https://github.com/docker/compose/releases/download/1.25.5/docker-compose-$(uname -s)-$(uname -m)" -o /usr/local/bin/docker-compose
sudo chmod +x /usr/local/bin/docker-compose
sudo service docker start
sudo /usr/local/bin/docker-compose build
sudo /usr/local/bin/docker-compose up
```

I tried doing the normal install but kept getting stuck in random laravel issues and thought just using the Docker would be simpler.