Update README support DB_PORT

Author: thohng

README.md

@@ -101,6 +101,7 @@ services:
       - PGID=1000
       - APP_URL=
       - DB_HOST=bookstack_db
+      - DB_PORT=3306
       - DB_USER=bookstack
       - DB_PASS=<yourdbpass>
       - DB_DATABASE=bookstackapp
@@ -137,6 +138,7 @@ docker run -d \
   -e PGID=1000 \
   -e APP_URL= \
   -e DB_HOST=<yourdbhost> \
+  -e DB_PORT=<yourdbport> \
   -e DB_USER=<yourdbuser> \
   -e DB_PASS=<yourdbpass> \
   -e DB_DATABASE=bookstackapp \
@@ -157,6 +159,7 @@ Container images are configured using parameters passed at runtime (such as thos
 | `-e PGID=1000` | for GroupID - see below for explanation |
 | `-e APP_URL=` | for specifying the IP:port or URL your application will be accessed on (ie. `http://192.168.1.1:6875` or `https://bookstack.mydomain.com` |
 | `-e DB_HOST=<yourdbhost>` | for specifying the database host |
+| `-e DB_PORT=<yourdbport>` | for specifying the database port if not default 3306 |
 | `-e DB_USER=<yourdbuser>` | for specifying the database user |
 | `-e DB_PASS=<yourdbpass>` | for specifying the database password |
 | `-e DB_DATABASE=bookstackapp` | for specifying the database to be used |