Added a command in the startup script to make sure the logging folder…

… exist (ref: knative/serving#3809)

README.md

@@ -1,4 +1,6 @@
-By default this custom `NGINX` docker image behave like a regular stock NGINX image. However, it can be customized in a couple of ways:
+By default this custom `NGINX` docker image behave like a regular stock NGINX image. However, it also supports a few customizations if and when needed. 
+
+The customizations supported are:
 
 - if the `INDEX_HTML_CONTENT` system variable is passed to the container, the image will replace the `index.html` file with the content of the variable
 - if the `HTTP_PORT` system variable is passed to the container, the image will configure the `/etc/nginx/conf.d/default.conf` to listen on the port specified in the variable 
@@ -17,4 +19,4 @@ The command below runs a stock nginx image replacing the default `index.html` fi
 docker run --rm -d -e INDEX_HTML_CONTENT="My custom web site" -e HTTP_PORT=8080 -p 8080:8080 mreferre/nginx-custom-site:latest
 ```
 
-The two variables can be used independently. 
+The variables can be used independently. 

startup.sh

@@ -1,6 +1,9 @@
 #!/bin/bash
-#!/bin/bash
+# this is a list of all variables supported
 if [ "${INDEX_HTML_CONTENT}" ]; then echo $INDEX_HTML_CONTENT > /usr/share/nginx/html/index.html; fi
 if [ "${HTTP_PORT}" ]; then sed -i "s/listen       80/listen       ${HTTP_PORT}/g" /etc/nginx/conf.d/default.conf; fi
+# the following is required in some rare circumstances where a volume is mounted at /var/log (ref: https://github.com/knative/serving/issues/3809)  
+mkdir -p /var/log/nginx
+# this starts the nginx service  
 nginx -g "daemon off;"