Dockers: external drive as shared volume in OSX, Permissions - macos

Question

First let me describe the system that I'm working on.
Intro
I've got a Mac with a small SSD drive. The system contains different
tools for data analysis that run as separated dockers.
Docker for OSX does not run directly on the system and it requires a
linux-based Virtual Machine which runs the dockers internally.
Some of these tools store huge amounts of data (several TB), which means that the internal HDD would not be enough so we need to store the data in an external media.
Therefore, for each docker it would be interesting to separate the data storage (databases) and the system files (OS + tool sources). Databases must be stored at the external HDD.
What I've done is:
Create a shared directory at the external HDD between OSX and the Linux-based VM (host for the dockers). Let's say that this directory is mounted at "/data" within the Linux VM.
Then, when creating a docker, use the "/data" directory from host as a volume mounted at the "/data" directory of the docker. If I ssh to one of the created dockers and list the content of the "/data" dir, I can see the content of the external HDD. So that works.
Here is when the problem appears, and it is related with PERMISSIONS.
PROBLEM
Considering that:
At the OSX our user is peterfoo
At the dockers the main user is also peterfoo, but we may have secondary users for the different system services. For example, if we install MYSQL, the MySQL service will be executed by default by the user mysql and all the files will belong to this user.
I want to change the MySQL data directory to enforce that all the data is stored at the "/data" directory (i.e. at the external HDD)
If I user the user peterfoo there is not problem: I can write files, read files, remove and create directories, etc. at the "/data" directory (external HDD) from the docker. However, using a different user the system does not allow writting, which means that when the mysql user tries to write/read data from the new location it fails and MySQL stops working.
SOLUTIONS?
I couldn't find a solution at this forum and I've being trying to fix this issue from yesterday.
The only solution that I found for the moment is a really naive one: run the MySQL (and other services) as the peterfoo user, instead of the correspoding one (mysql). This is not a critical issue but introduces a security problem: it is always recomendable to use specific users for the system services to keep control of the access to data.
Any idea?
Thanks!!
More info
If I run ls -l to /data directory, I get:
drwxr-xr-x 8 peterfoo staff 272B 17 may 09:48 app1-data/
drwxr-xr-x 3 peterfoo staff 102B 25 may 12:38 app2-data/
drwxr-xr-x 7 peterfoo staff 238B 25 abr 10:35 app3-data/
If I try to change the ownership of the directory to mysql, it doesn't work and keeps peterfoo.
$ sudo chown mysql:mysql /data/app1-data
$ ls -l /data | grep app1
drwxr-xr-x 8 peterfoo staff 272B 17 may 09:48 app1-data/
If also tries changing the rwx permissions to 777 and then using a secondary user (let's say user2) can create dirs and files but once created, the ownership changes automatically to peterfoo, and the created subdirectories become unwritable to user2.
$ sudo chmod 777 /data/app1-data
$ ls -l /data | grep app1
drwxrwxrwx 8 peterfoo staff 272B 17 may 09:48 app1-data/
$ su user2
$ touch test.txt
$ mkdir /data/app1-data/test
$ ls -l /data/app1-data/
drwxrwxrwx 1 peterfoo staff 204 May 25 12:38 ./
drwxrwxr-x 1 peterfoo staff 510 May 24 14:29 ../
-rw-r--r-- 1 peterfoo staff 0 May 25 08:07 test.txt
drwxr-xr-x 1 peterfoo staff 68 May 25 12:38 test/

Answer 1

Uhm let me get this straight:
you got a virtual machine for linux to wich you attached part of the external storage; you then mounted that storage at /data in linux system.
created a docker container with a volume at /data inside the container mounted at /data inside the linux vm.
if i got that right the docker should automatically have permission to write in /data at linux vm and the only problems i can think of are the permission that are set inside the docker at the /data directory.
if my hunch is right when you ssh inside the docker and ls -l at / you will find owner is peterfoo and lacking permission for other users