WORDPRESS_VERSION doesn't reliably work

[mallorydxw]

Adding the following to the wordpress container in docker-compose.yml doesn't reliably work:

    environment:
      WORDPRESS_VERSION: "4.9.1"

I just tested with the main branch and everything worked the first time, but it didn't work after that.

[snim2]

We'll need to try and reproduce this error before deciding whether the issue should be kept open...