Docker dev environment: Automate deployment of a debug build of Apps plugin

[mickmister]

For example, some Mattermost config settings to tweak in order to have a smoother dev experience. Basically anything that slows a developer down that can be remedied through automation.

• Have a default set of credentials, or some setup that easily allows re-deploy of the plugin. We can use mmctl with local mode to create this on container creation.
• Add a make deploy-dev command which would automatically use the credentials for the docker compose environment.
• Remove strict password requirements. This includes password length and special character requirements.

In the case where we want to instead use the mattermost-server repo instead of docker, possibly because we want the server and App to run in the same localhost network, we can provide a script to start the server with certain environment variables to make developing Apps more seamless.