elaborate on usage

Author: ChristianKniep

README.md

@@ -1,2 +1,75 @@
 # init-plain
  Script to provide an entrypoint and helpers for the plain docker image series. 
+
+## entrypoint.sh
+
+The script is supposed to be specified as `ENTRYPOINT` for a given Docker Image.
+E.g. within [qnib/alplain-init](https://github.com/qnib/alplain-init).
+
+It will iterate over the executables within `/opt/qnib/entry` and either execute or source them.
+
+```
+#!/bin/bash
+set -e
+
+for x in $(find /opt/qnib/entry/ -type f -perm /u+x |sort);do
+     echo "> execute entrypoint '${x}'"
+     if [[ "$x" == *.env ]];then
+         source ${x}
+     else
+         ./${x}
+     fi
+done
+
+if [ "X${ENTRY_USER}" != "X" ];then
+  exec su -s /bin/bash -c "$@" ${ENTRY_USER}
+else
+  exec "$@"
+fi
+```
+
+Once this is done, the command defined with `CMD` will be executed and the process control is handed over - in case an `$ENTRY_USER` is specified under a user other then `root`.
+
+## `/opt/qnib/entry`
+
+### Share log-socket
+
+The first script (`00-`) executed will search for a log-device of an underlying syslog server and create a symlink to `/dev/log`, so that the container can naturally use this socket.
+
+```
+$ cat /opt/qnib/entry/00-logging.sh
+#!/bin/bash
+
+if [ ! -z ${LOGNAME} ] && [ -S /shareddev/${LOGNAME}/log ];then
+    ln -s /shareddev/${LOGNAME}/log /dev/log
+    logger -s "Created container specific symlink to syslog container"
+elif [ -S /shareddev/log ];then
+    ln -s /shareddev/log /dev/log
+    logger -s "Created symlink from syslog container"
+fi
+```
+
+### environment variables
+
+In case environment variables should be sourced before the main process starts, the prefix `.env` should be used.
+
+This exemplary script sources docker-secrets as environment variables.
+
+```
+$ cat /opt/qnib/entry/10-docker-secrets.env
+#!/bin/bash
+
+########
+## Check for /run/secrets and expose them as ENV variables
+mkdir -p /run/secrets/
+if [ -d /run/secrets/ ];then
+    for sec in $(ls /run/secrets/);do
+        KEY=$(echo ${sec} |tr '[:lower:]' '[:upper:]' |sed -e 's/-/_/g')
+        echo "[II] Set environment variable ${KEY} from '/run/secrets/${sec}'"
+        declare "$KEY=$(cat /run/secrets/${sec})"
+        export $KEY
+    done
+else
+    echo "[II] No /run/secrets directory, skip step"
+fi
+```