feat: 💥 Allow creating multiple instances of app containers

[CLHatch]

Purpose
Allow creating multiple instances of built-in apps without using an override.

Approach
Replace all references of the app's name in templates in ~/.docker/compose/.apps to <appname> and <APPNAME>.
On run, copy the folders of apps being used to ~/.docker/compose/.instances, using folder names similar to below.:

.instances/radarr
.instances/radarr__radarr4k

The "appname" variables in the files in these folders will be replaced by the folder name (<appname> in lower case, <APPNAME> in upper case).

We will then use the .instances folder instead of the .apps folder when creating the .env file, and when calling Docker Compose.

The service names and container names of the instances will match the folder names, to avoid possible conflicts with names of other built-in apps (in the above example, both service name and container name of the radarr4k instance would be radarr__radarr4k). The user could then choose to change the container name to something else if they wish in the .env file (for example, RADARR__RADARR4K__CONTAINER_NAME='radarr4k'). This would allow the user to use a shorter name for Docker commands and network references (for example, docker stop radarr4k and http://radarr4k:7878 instead of docker stop radarr__radarr4k and http://radarr__radarr4k:7878). The service name will remain unchanged.

The format of the app variable names will be changed slightly, going from a single underscore before the start of the variable part to two underscores. We will attempt to migrate existing old variables to the new naming when they are encountered. The general format is <APPNAME>__<VARIABLE> or <APPNAME>__<INSTANCENAME>__<VARIABLE> (i.e. RADARR__CONTAINER_NAME or RADARR__RADARR4K__CONTAINER_NAME).

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