unnecessarily restricting operations on the shared files, Docker Desktop runs
the file sharing service (`virtiofsd`) inside a user namespace (see
`user_namespaces(7)`) with UID and GID mapping configured. As a result Docker
Desktop relies on the host being configured to enable the current user to use
subordinate ID delegation. For this to be true `/etc/subuid` (see `subuid(5)`)
and `/etc/subgid` (see `subgid(5)`) must be present. Docker Desktop only
supports subordinate ID delegation configured via files. Docker Desktop maps the
current user ID and GID to 0 in the containers. It uses the first entry
corresponding to the current user in `/etc/subuid` and `/etc/subgid` to set up
mappings for IDs greater than 0 in the containers.

| ID in container | ID on host                                                                       |
| --------------- | -------------------------------------------------------------------------------- |
| 0 (root)        | ID of the user running Docker Desktop (e.g. 1000)                                            |
| 1               | 0 + beginning of ID range specified in `/etc/subuid`/`/etc/subgid` (e.g. 100000) |
| 2               | 1 + beginning of ID range specified in `/etc/subuid`/`/etc/subgid` (e.g. 100001) |
| 3               | 2 + beginning of ID range specified in `/etc/subuid`/`/etc/subgid` (e.g. 100002) |
| ...             | ...                                                                              |

If `/etc/subuid` and `/etc/subgid` are missing, they need to be created.
Both should contain entries in the form -
`<username>:<start of id range>:<id range size>`. For example, to allow the current user
to use IDs from 100 000 to 165 535:

```console
$ grep "$USER" /etc/subuid >> /dev/null 2&>1 || (echo "$USER:100000:65536" | sudo tee -a /etc/subuid)
$ grep "$USER" /etc/subgid >> /dev/null 2&>1 || (echo "$USER:100000:65536" | sudo tee -a /etc/subgid)
```

To verify the configs have been created correctly, inspect their contents:

```console
$ echo $USER
exampleuser
$ cat /etc/subuid
exampleuser:100000:65536
$ cat /etc/subgid
exampleuser:100000:65536
```

In this scenario if a shared file is `chown`ed inside a Docker Desktop container
owned by a user with a UID of 1000, it shows up on the host as owned by
a user with a UID of 100999. This has the unfortunate side effect of preventing
easy access to such a file on the host. The problem is resolved by creating
a group with the new GID and adding our user to it, or by setting a recursive
ACL (see `setfacl(1)`) for folders shared with the Docker Desktop VM.

{{< /accordion >}}

### Where does Docker Desktop store Linux containers?

Docker Desktop stores Linux containers and images in a single, large "disk image" file in the Linux filesystem. This is different from Docker on Linux, which usually stores containers and images in the `/var/lib/docker` directory on the host's filesystem.

#### Where is the disk image file?

To locate the disk image file, select **Settings** from the Docker Desktop Dashboard then **Advanced** from the **Resources** tab.

The **Advanced** tab displays the location of the disk image. It also displays the maximum size of the disk image and the actual space the disk image is consuming. Note that other tools might display space usage of the file in terms of the maximum file size, and not the actual file size.