Split .docker config into separate directories following platform conventions

[Mewsen]

Split the current .docker directory into separate directories for configuration, data, cache, and runtime, following XDG conventions on Unix and sensible platform-specific paths on Windows and macOS.

Example:

Purpose	Unix XDG	Unix w/o XDG	Unix System Level	Windows User Level	Windows System Level	macOS User Level	macOS System Level
Configuration	$XDG_CONFIG_HOME/docker	~/.docker	/etc/docker/cli	%APPDATA%\Docker	%PROGRAMDATA%\Docker	~/Library/Preferences/Docker	/Library/Preferences/Docker
Data	$XDG_DATA_HOME/docker	~/.docker	/usr/local/share/docker	%LOCALAPPDATA%\Docker	%PROGRAMDATA%\Docker	~/Library/Application Support/Docker	/Library/Application Support/Docker
Cache	$XDG_CACHE_HOME/docker	~/.docker	/var/cache/docker	%LOCALAPPDATA%\Docker\Cache	%PROGRAMDATA%\Docker\Cache	~/Library/Caches/Docker	/Library/Caches/Docker
Runtime	$XDG_RUNTIME_DIR/docker	~/.docker	/run/docker/cli	%LOCALAPPDATA%\Docker\Run	%PROGRAMDATA%\Docker\Run	?	?

Are these paths sensible, do they crash in any way with the docker daemon?

Any concerns around backward compatibility?

Would love to create a PR.

related: #2738, #2423