Azure Web App usage of WEBAPP_STORAGE_HOME variable in docker-compose - azure

Question

I'm trying to set up a web app that has persistent storage via file share to a storage account.
I'm following various guides from Microsoft docs and I managed to do most of it, my app has persisted storage. But now, what I wanna do, I want to map volumes to my storage account.
I saw that there is this variable ${WEBAPP_STORAGE_HOME} that I can use in my docker-compose.
My question is, what is the value of this variable? The docs state:
${WEBAPP_STORAGE_HOME} is an environment variable in App Service that is mapped to persistent storage for your app.
I find this a little vague. Does it know automatically to map my path mappings? What if I have multiple path mappings? Should I set the value in the App Settings in the Configuration blade? If so, what do I need to specify, the mount path?
Besides that, I saw that it is used as following:
version: '3.3'
services:
wordpress:
image: mcr.microsoft.com/azuredocs/multicontainerwordpress
volumes:
- ${WEBAPP_STORAGE_HOME}/site/wwwroot:/var/www/html
ports:
- "8000:80"
restart: always
I'm used with named volumes in docker-compose. I figure there is no need to specify something like that?
UPDATE
After #Jason Pan's answer, I tried to play with the mount a little bit.
I succeeded in having persisted storage on the App Service with the following docker-compose:
# ... lines skipped for brevity
volumes:
- mysql-data:/var/lib/mysql
volumes:
mysql-data:
driver: local
But I want to persist the data on a Storage Account. I saw that this is possible via: AppService/Configuration/Path Mappings.
My Docker Compose
# ...
volumes:
- MyMountedPath:/var/lib/mysql
In this docker-compose file, I have my app and a MySQL image: mysql:8 to be exact.
I mounted a path as follows:
Name: MyMountedPath; Mounted Path: /usr/local/mysql; Type: Azure Files ....
And I get the following error in the logs:
2021-04-08T11:02:06.790578922Z chown: changing ownership of '/var/lib/mysql/': Operation not permitted
2021-04-08T11:02:12.785079208Z 2021-04-08 11:02:12+00:00 [Note] [Entrypoint]: Entrypoint script for MySQL Server 8.0.23-1debian10 started.
Since it worked with the first approach, I suspect that there are some issues with the way my Path mapping is defined. This led me to even more questions:
Does my Mount Path need to exist on the App Service File system? If no, can I define something like: /foo/bar ?
If I have a Mount Path name MyMountedPath, can I specify in the docker-compose file something like
volumes:
- MyMountedPath/foo:/something
Basically, navigating in the mounted path?
Do I need to mount this path in the File Storage in my Storage Account? Or will the App Service create this path when it will need to store something?

Answer 1

Example:
In the App Service properties, I mounted an Azure File Share and gave the name MyExternalStorage
In the docker compose configuration I have to set
volumes:- MyExternalStorage:/var/www/html/contao
Thanks for TeddyDubois29's answer, hope it also can help you.
Web App Docker Compose Persistent Storage