[Suggestion] nopull or cached-image flag

[lethedata]

Is your feature request related to a problem? Please describe.
if container_always_pull=1 or DBX_CONTAINER_ALWAYS_PULL=1 there's not a flag to overwrite these and still use local cache. The use case for overwriting these would be that one always wants to pull but there is a locally build image that needs to be tested and without a flag to disable it distrobox always tries to pull.

Describe the solution you'd like
Have a --nopull flag to disable pulling images all together (new behavior) and only use the local cache OR a --cached-image flag to use the cached image if available locally otherwise pull (same as default behavior).

Describe alternatives you've considered
Setting DBX_CONTAINER_ALWAYS_PULL=0 then calling distrobox works but seems like a flag would be useful.

Additional context
The --cached-image flag is probably easier to implement as it's just another case statement. I have an untested solution on my fork that might work for this.