Merge pull request #27 from utgwkk/notice-for-docker

doc: notice for Docker environments

Author: utgwkk

README.md

@@ -32,5 +32,7 @@ jobs:
     - run: pytest
 ```
 
+If your test is running in a Docker container, you have to install this plugin and manually set `GITHUB_ACTIONS` environment variable to `true` inside of Docker container. (For example, `docker-compose run --rm -e GITHUB_ACTIONS=true app -- pytest`)
+
 ## Screenshot
 [![Image from Gyazo](https://i.gyazo.com/b578304465dd1b755ceb0e04692a57d9.png)](https://gyazo.com/b578304465dd1b755ceb0e04692a57d9)