Fix #169: Explicitly state different UIDs/GIDs across distros

This is a simple documentation update to notify users that UIDs/GIDs may
(and in fact are) different across distros.

The documentation now reflects the last known state od UIDs/GIDs.
Startup now announces specific UIDs/GIDs used to make sure people are
aware of any future changes.

README.md

@@ -52,6 +52,7 @@ Feel free to pick your favourite distro.
   - [Extending the image](#extending-the-image)
     - [Using custom init scripts](#using-custom-init-scripts)
   - [Security](#security)
+    - [UIDs/GIDs numbers](#uidsgids-numbers)
   - [Quick how-tos](#quick-how-tos)
     - [Relaying messages through your Gmail account](#relaying-messages-through-your-gmail-account)
     - [Relaying messages through Google Apps account](#relaying-messages-through-google-apps-account)
@@ -684,7 +685,22 @@ postconf -e "address_verify_negative_cache=yes"
 ## Security
 
 Postfix will run the master proces as `root`, because that's how it's designed. Subprocesses will run under the `postfix`
-account which will use `UID:GID` of `100:101`. `opendkim` will run under account `102:103`.
+and `opendkim` accounts.
+
+### UIDs/GIDs numbers
+
+While I cannot guarantee IDs (they are auto-generated by package manages), they tend to be fairly consistent across 
+**specific distribution**. Please be aware of this if you are switching images from Alpine to Debian to Ubuntu or
+back.
+
+At the last check, images had the following UIDs/GIDs:
+
+| Service    | Debian (`UID/GID`) | Ubuntu (`UID/GID`) | Alpine (`UID/GID`) |
+|------------|--------------------|--------------------|--------------------|
+| `postfix`  | `100:102`          | `101:102`          | `100:101`          |
+| `opendkim` | `101:104`          | `102:104`          | `102:103`          |
+
+Please check the notification information on startup.
 
 ## Quick how-tos
 

scripts/common-run.sh

@@ -1,6 +1,8 @@
 #!/usr/bin/env bash
 
 announce_startup() (
+	local postfix_account opendkim_account
+
 	DISTRO="unknown"
 	[ -f /etc/lsb-release ] && . /etc/lsb-release
 	[ -f /etc/os-release ] && . /etc/os-release
@@ -10,6 +12,11 @@ announce_startup() (
 		DISTRO="${ID}"
 	fi
 	echo -e "${gray}${emphasis}★★★★★ ${reset}${lightblue}POSTFIX STARTING UP${reset} ${gray}(${reset}${emphasis}${DISTRO}${reset}${gray})${emphasis} ★★★★★${reset}"
+
+	postfix_account="$(cat /etc/passwd | grep -E "^postfix" | cut -f3-4 -d:)"
+	opendkim_account="$(cat /etc/passwd | grep -E "^opendkim" | cut -f3-4 -d:)"
+
+	notice "System accounts: ${emphasis}postfix${reset}=${orange_emphasis}${postfix_account}${reset}, ${emphasis}opendkim${reset}=${orange_emphasis}${opendkim_account}${reset}. Careful when switching distros."
 )
 
 setup_timezone() {