Add new user to the same groups as is used by Debian Installer

This allows dropping the modifications that mkimage.sh does to /etc/adduser.conf. Changing that file is disruptive for users every time that adduser upgrades change the file because they have to investigate a diff of the proposed changes.

For example they will see diffs like this:

--- /etc/adduser.conf   2025-05-08 13:26:50.000000000 +0100
+++ /etc/adduser.conf.dpkg-new  2026-03-28 09:16:28.000000000 +0000
@@ -76,11 +76,6 @@
 # Default: SYS_DIR_MODE=0755
 #SYS_DIR_MODE=0755
 
-# If set to a nonempty value, new users will have quotas copied
-# from that user with `edquota -p QUOTAUSER newuser'
-# Default: QUOTAUSER=""
-#QUOTAUSER=""
-
 # Non-system user- and groupnames are checked against this regular
 # expression. RFC8265 would allow all codepoints U+0021 through U+007E
 # meaning: !"#$%&'()*+,-./:;<=>?@[\]^_`{|}~ plus digits, lower case and
@@ -106,11 +101,10 @@
 # list of groups that new non-system users will be added to
 # if ADD_EXTRA_GROUPS is non-zero or set on the command line.
 # Default: EXTRA_GROUPS="users"
-EXTRA_GROUPS=audio cdrom dip floppy video plugdev netdev
+#EXTRA_GROUPS="users"
 
 # Setting this to something other than 0 will cause adduser to add
 # newly created non-system users to the list of groups defined by
 # EXTRA_GROUPS.
 # Default: ADD_EXTRA_GROUPS=0
-ADD_EXTRA_GROUPS=1
-
+#ADD_EXTRA_GROUPS=0