fix: use exact match for USER_NAME in /etc/passwd check

The grep pattern `^${USER_NAME}` incorrectly matches usernames that
are prefixes of existing users. For example, USER_NAME=b matches the
'bin' user, causing the container to halt with a false positive.

Adding a colon after USER_NAME ensures exact username matching:
`^${USER_NAME}:` only matches the exact username field.

closes #118

Author: Brendan DeBeasi

root/etc/s6-overlay/s6-rc.d/init-adduser/run

@@ -1,7 +1,7 @@
 #!/usr/bin/with-contenv bash
 # shellcheck shell=bash
 
-if [[ ! -f "/usermod.done" ]] && [[ -n "${USER_NAME}" ]] && [[ "${USER_NAME}" != "abc" ]] && grep -q "^${USER_NAME}" /etc/passwd; then
+if [[ ! -f "/usermod.done" ]] && [[ -n "${USER_NAME}" ]] && [[ "${USER_NAME}" != "abc" ]] && grep -q "^${USER_NAME}:" /etc/passwd; then
     echo "*** USER_NAME cannot be set to an user that already exists in /etc/passwd. Halting init. ***"
     sleep infinity
 else