1. Ubuntu WSL2 must be already installed in C: system drive and user should be able to call windows binaries like wsl.exe from bash.

2. We will install Alpine WSL2 distro in an external partition/disk:

    Download an Alpine ZIP file from here:
    https://github.com/yuk7/AlpineWSL/releases/

    Extract the files in an external partition/disk. (for example D:\Alpine)
    Make sure WSL2 is enabled by default (wsl.exe --set-default-version 2)
    Inside the Alpine folder run `winpty Alpine.exe` to install the Distro. An ext4.vhdx file will be created in that same folder.
    Run `winpty Alpine.exe` again.
    Now from the Alpine terminal we will create same user as Ubuntu:
        `addgroup -g 1000 nevmerzhitsky`
        `adduser -u 1000 -G nevmerzhitsky nevmerzhitsky`
    This will create the /home/nevmerzhitsky folder.
    
3. On Ubuntu edit the ~/.profile file and add:

# mount external /home folder from Alpine distro
if [ ! -d /mnt/wsl/Alpine ]; then
    mkdir /mnt/wsl/Alpine
    wsl.exe -d Alpine mount --bind /home/onoma /mnt/wsl/Alpine/
fi

4. We will close the VM with:
    wsl.exe --shutdown

5. Next time Ubuntu distro is launched it will automount the /home/onoma folder from Alpine distro to /mnt/wsl/Alpine in Ubuntu with all
the benefits and speed of ext4 mounted folders.

How it works
---------------
WSL2 runs a single lightweight VM that supports multiple distros running on the same Linux kernel. 
Every time a distro is launched, its vhdx file is attached automatically to the VM as a /dev/sdc device.
Thanks to the nature of WSL interop we can launch another distro and inmediately close it from inside bash.

The folder /mnt/wsl is an undocumented WSL2 feature which is a tmpfs special folder used by applications as Docker for Desktop.
Everything mounted in that folder will also appear in every running distro under /mnt/wsl/. Just make sure you have file permissions to 
access that directory. In my case I'm using user "onoma" on both Ubuntu and Alpine.