Toolbox does currently not work with SELinux's 'confined user' accounts (affected: Fedora and CentOS, maybe also RHEL)

**Describe the bug**
Once SELinux's 'confined user' [1] accounts are used, toolbox does no longer work on Fedora and CentOS:

The issue is not logged in journalctl, neither at user nor at root, only the immediate terminal output of toolbox.

Three examples of confined user accounts:

confined user: staff_u (Fedora)
```
bash-5.2$ id -Z
staff_u:staff_r:staff_t:s0-s0:c0.c1023
bash-5.2$ toolbox enter
exec: Permission denied
Error:
```

confined user: staff_u with sysadm_r through sudo (Fedora)
```
bash-5.2$ id -Z
staff_u:sysadm_r:sysadm_t:s0-s0:c0.c1023
bash-5.2$ toolbox enter
Error: failed to initialize container fedora-toolbox-38
```

-> Only `sysadm_u:sysadm_r:sysadm_t` (in the test case, `xdm_sysadm_login` was set true) seems to work on Fedora, but I guess for several use cases, this is not appropriate, especially on CentOS/RHEL -> I have not tested `sysadm_u:sysadm_r:sysadm_t` on CentOS, but staff_u is already the highest privilege after sysadm_u, so CentOS needs at least sysadm_u.

confined user: staff_u (CentOS)
```
[user@localhost ~]$ id -Z
staff_u:staff_r:staff_t:s0
[user@localhost ~]$ toolbox enter
Error: invalid entry point PID of container rhel-toolbox-latest
```

In all cases, once the user confinement is removed by `unconfined_u` (or alternatively, at least on Fedora, set to sysadm_u with `xdm_sysadm_login`=true), toolbox works again properly.

I have not tested with RHEL.

If toolbox is not intended to work with confined user accounts, feel free to close the ticket.

[1] elaboration of SELinux user confinement, e.g., on https://access.redhat.com/documentation/de-de/red_hat_enterprise_linux/8/html/using_selinux/managing-confined-and-unconfined-users_using-selinux

**Steps how to reproduce the behaviour**
Applicable to Fedora 38 and CentOS:
1. Confine user with user_u or staff_u (including staff_u that can become sysadm_r through `sudo`)
2. (if applicable: logout, then:) Login (if confinement is added or changed, the user has to logout and login again after the change in order to enable the new setting)
3. do `toolbox enter` -> issue occurs
4. Set user account to sysadm_u or unconfined_u
5. Logout+Login
6. do `toolbox enter` -> toolbox works again

**Expected behaviour**
Toolbox should work in confined user accounts.

**Actual behaviour**
Toolbox does not work in confined user accounts.

**Output of `toolbox --version` (v0.0.90+)**
toolbox version 0.0.99.4

**Toolbox package info (`rpm -q toolbox`)**
toolbox-0.0.99.4-1.fc38.x86_64

**Output of `podman version`**
```
Client: Podman Engine
Version: 4.5.1
API Version: 4.5.1
Go Version: go1.20.4
Built: Fri May 26 19:58:48 2023
OS/Arch: linux/amd64
```

**Podman package info (`rpm -q podman`)**
podman-4.5.1-1.fc38.x86_64

**Info about your OS**
Fedora 38 KDE Spin, up to date as of today, only default repos, no additional software/modules
CentOS Stream 9, up to date as of today, only default repos, no additional software/modules

**Additional context**
I tested it today the first time.
For testing, I used both on Fedora and CentOS the default image that is deployed when starting with `toolbox enter`.

Start with the `toolbox enter` entry point and reproduce the reported failures under `staff_u` on Fedora and CentOS. Compare the immediate toolbox output and SELinux contexts for the working and confined-user cases; done means toolbox can initialize and enter its default container for the documented confined accounts.

Written by the indexing model from the issue text.