Use socket mode events

[surdaft]

Slack have deprecated the RTM API and recommend using the socket mode API instead.

https://api.slack.com/rtm
https://api.slack.com/apis/connections/socket

New Slack apps may not use any Real Time Messaging API method
For most applications, Socket Mode is a better way to communicate with Slack.

I have altered the documentation and updated the code to allow the use of this new API. In addition I added a docker build action to allow the use of the memo system through a precompiled docker image.

This is a backwards breaking change due to the requirement of an app token to create the socket connection.

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Any feedback on this is very much recommended, I intend to add some QOL updates as time progresses, as I enjoy using this convenient memo system.

[CLAassistant]

All committers have signed the CLA.

[surdaft]

The dockerfile is only vaguely tested due to working from a private corporation network, and so the SSL is failing to validate. External compilation works fine though and so in theory it should be all good in there. Would be worth a test though.