Setting custom startup command leaves no option to remove it

[vilhelmprytz]

Is your feature request related to a problem? Please describe.
If you decide to use a custom startup command, there is no way of resetting to the default startup command. Instead, you have to look the default command for that specific image and then manually set it.

Describe the solution you'd like
Some kind of way to reset back to the default startup command easily.

Describe alternatives you've considered
None.

Additional context
None.