Configuring DKIM Error

The system and versions:
I am running ODK Central on Linux Ubuntu 18.04 LTS.
ODK Central versions: client (v0.8.0), server (v0.8.1).

The problem is this:
Users are not receiving emails anymore. I noticed this after trying to reset a user's password and creating an account. Users would receive mail just fine once I launched Central. It would be marked as spam on some mail servers, and as normal mail on others.
Now they no longer receive emails.

What have I tried:
I tried using the Configuring DKIM guide, and I ran into 2 issues.
i) Instruction 4 is pretty unclear, which locations are being referred to for the TXT DNS records. I assume on a Nameserver perhaps?

ii) Instruction 5, the command 'docker-compose build' results in a 'mail uses an image, skipping' error. So then I tried removing the mail server and building again, but it results in the same error.

Please help with resolving this.

Central v0.8 is pretty old now, please upgrade to the latest version of Central (v1.1.2).

As to the issue at hand, see the Users aren't receiving emails section of our documentation.

@yanokwa Did as you said and tried updating central, several services, namely postgres, pyxform, enketo_redis, enketo_redis_main all give the message " uses an image, skipping".

Thus it seems the new features listed by @Matthew_White the newer version don't show up once the build is done.

Nevermind. The updates seem to be working fine now. Refreshed the page and it seems to be working okay. I'll troubleshoot continue the mail config and return with results.