SMTP Mail sending in docker container and documentation for sending mail with msmtp

[Krafting]

Pre-Request Checklist

• Passes rubocop code analysis (try rubocop --auto-correct)
• Tests added or adapted (try rake test)
• Changes are reflected in the documentation
• User-visible changes appended to CHANGELOG.md

Description

Added the package msmtp to the docker container to add the ability to send mail using SMTP when using hooks and docker, also added relevent documentation.

I'm not that happy with the whole symlink from .config/oxidized to the homedir for the .msmtprc file, but having config all in the same place is easier to manage and it does work.

Please give feedback!

Documentation might also be badly worded or not understandable enough, feel free to give me feedback.

Thanks everyone involved in this project!

Closes issue #2574

Depends on PR #2813 (Fixes #2769)

[electrocret]

I don't believe PR #2813 is absolutely needed (Of course it couldn't hurt). I was able to get my container, which is running latest, to send failed backup emails using the changes in this PR.

[Krafting]

I don't believe PR #2813 is absolutely needed (Of course it couldn't hurt). I was able to get my container, which is running latest, to send failed backup emails using the changes in this PR.

Maybe it changed since I last tested it, will need to retry.

Also, if you have any feedback, especially about the documentation and the whole process to add mail notification it would help a lot!

[electrocret]

Nah, it was extremely straightforward what I did. I executed all the new commands in the Dockerfile on my existing container running the latest oxidized, then added the documented lines to my config. The only part where I had to do a little bit of troubleshooting was getting msmtp configured correctly. So maybe add a link to an msmtp guide? However that's really out of scope of Oxidized.

I also think I'm not affected by #2813, cause I haven't noticed it in my logs despite running 0.29.1.

On an tangentially related topic - I had to disable this cause we had more nodes failing than we thought and it was spamming us, but still not an oxidized issue, it's a node config issue. 😆