[FEATURE] Make --squash argument optional

[picsel2]

Is your feature request related to a problem? Please describe.

When building "from scratch" this action always squashes all layers including those from the base image.
This is undesirable for me since it prevents efficient reuse of the base layers.

Describe the solution you'd like

A simple boolean input parameter that allows to disable squashing.

Describe alternatives you've considered

None other.

Additional context

One problem might be, that such a flag might be also desirable for Containerfile based builds.
From the looks it seems to me that Containerfile builds are not squashed at the moment, while scratch builds currently always squash.
So deciding for a common default behaviour will break either one or the other method.