Replace old environment variable name

pica-etherpad/entrypoint.sh

@@ -24,7 +24,7 @@ fi
 
 # Check if database already exists
 RESULT=`mysql -u${ETHERPAD_DB_USER} -p${ETHERPAD_DB_PASSWORD} \
-	-h${MYSQL_PORT_3306_TCP_ADDR} --skip-column-names \
+	-h${ETHERPAD_DB_HOST} --skip-column-names \
 	-e "SHOW DATABASES LIKE '${ETHERPAD_DB_NAME}'"`
 
 if [ "$RESULT" != $ETHERPAD_DB_NAME ]; then