Allow containers to have their own default shells different from the host

**Describe the bug**
Shell does not change from bash to zsh with neither chsh or usermod -s

**Steps how to reproduce the behaviour**
```
toolbox create --distro fedora --release 33
toolbox enter --release 33
echo $SHELL
sudo dnf install zsh
sudo usermod -s /bin/zsh alvar (or chsh, using `util-linux-user`)
exit
toolbox enter --release 33
echo $SHELL
```

**Expected behaviour**
$SHELL changes from bash to zsh

**Actual behaviour**
$SHELL stays at bash

**Output of `toolbox --version` (v0.0.90+)**
`toolbox version 0.0.99.1`

**Toolbox package info (`rpm -q toolbox`)**
`toolbox-0.0.99.1-1.fc34.x86_64`

**Output of `podman version`**
```
Version: 3.1.0
API Version: 3.1.0
Go Version: go1.16
Built: Tue Mar 30 15:29:36 2021
OS/Arch: linux/amd64
```

**Podman package info (`rpm -q podman`)**
`podman-3.1.0-1.fc34.x86_64`

**Info about your OS**
Fedora Silverblue 34