Use redis as IMAP- and session-cache

[sholl]

What type of PR?

Feature

What does this PR do?

This PR uses the already used redis-deployment for storing roundcube IMAP- and session-cache in order to speed up the Web-UI of roundcube.

Related issue(s)

• Fixes Use redis from within roundcube webmail #2943

Prerequisites

Before we can consider review and merge, please make sure the following list is done and checked.
If an entry in not applicable, you can check it or remove it from the list.

• [X ] In case of feature or enhancement: documentation updated accordingly
• Unless it's docs or a minor change: add changelog entry file.

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• CI-Done

[nextgens]

I don't want to sound obnoxious but have you tested it? What performance improvement is expected here?

My take is that it can't possibly work as is since the redis container is on the "default" network which is unreachable from the "webmail" network.

[sholl]

I don't want to sound obnoxious but have you tested it? What performance improvement is expected here?

Yes, I have tested this on my live-system, but with a slightly different network setup, so your hint with default-network is helpful. My redis-cluster runs in the default network, webmail as well.
I should reread the docs for testing changes locally...

My take is that it can't possibly work as is since the redis container is on the "default" network which is unreachable from the "webmail" network.

right.
You can decide what will be best for the project. Adopt this PR (perhaps also add redis to the webmail-network), or skip it entirely, since it can be (easily) added to the roundcube-config via the overrides. I could also add a hint in the docs

What is your preference?

[nextgens]

I personally think that documentation would be better. This is useful/required if you "scale up" the number of webmail instances... which arguably we do not support.

I remain to be convinced that on a single instance deployment it significantly changes anything.

[Deltachaos]

I am interested in this as well as it will increase HA adoption

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Merge conflict.