fix: remove docker from defaults, fix cliphist-image ordering cycle

Docker is a dev dependency, not a desktop environment default. Remove
from package list, archinstall config, services and README.

Fix systemd ordering cycle that prevented cliphist-image from starting:
cliphist-text had After=graphical-session.target which combined with
PartOf= and cliphist-image's After=cliphist-text created a cycle.

defaults/etc/systemd/user/cliphist-text.service

@@ -4,7 +4,6 @@
 [Unit]
 Description=Clipboard history manager (text)
 PartOf=graphical-session.target
-After=graphical-session.target
 
 [Service]
 Type=simple