Slack.event.invalid-token trying to configure Slack app

I am trying to configure the Slack integration as per the pinned post. When I try to configure the event URL, Slack tells me:

Your URL didn’t respond with the value of the challenge parameter.

In my Sentry logs, I see this:

13:04:03 [ERROR] sentry.integrations.slack: slack.event.invalid-token
x.x.x.x - - [15/Jul/2018:13:04:02 +0000] "POST /extensions/slack/event/ HTTP/1.1" 400 211 "-" "Slackbot 1.0 (+"

In /etc/sentry/config.yml I have:

slack.client-id: '…'
slack.client-secret: '…'
slack.verification-token: '…value from Slack verification token…'

I’ve tried regenerating the token but that didn’t help. Any ideas what I’m missing here?

When I try to add the workspace I get sent to a URL with a blank client_id so I suspect my config isn’t being loaded at all. I’m using the Docker image and I can verify that /etc/sentry/config.yml has the correct contents in the container.

Okay, I figured it out; config.yml was owned by root with 0400 perms, so the Sentry web process running as sentry could not read it. Is there a warning/error about this that I missed?