add -f flag to prune commands for docker compat

[knQzx]

summary

• adds -f/--force to image/container/volume/network prune as a no-op
• prints a warning to stderr when used, so the flag is not silently ignored
• unblocks docker automation scripts calling prune -f

test plan

• added CLI test: container prune -f succeeds, prints warning on stderr, normal output on stdout
• did all four prune variants since docker scripts typically call each; happy to narrow to just image prune if preferred

fixes #938

[saehejkang]

I don't think this is something we want here. Adding an option for a flag, to only just output it as a no-op is counter intuitive. However, I see the flag actually being useful, if it is properly implemented. This could be broken up into two separate issues:

• Update any prune commands with the --all flag to ask the user if they are sure they want to continue (like docker)
• Modifying this PR to bypass the prompt if the --force flag is used

What do you think?

[knQzx]

I don't think this is something we want here. Adding an option for a flag, to only just output it as a no-op is counter intuitive. However, I see the flag actually being useful, if it is properly implemented. This could be broken up into two separate issues:

• Update any prune commands with the --all flag to ask the user if they are sure they want to continue (like docker)
• Modifying this PR to bypass the prompt if the --force flag is used

What do you think?

i went with option 2 because that's what @ptone asked for in #938 - no-op with a warning. the issue scope is just docker compat so i kept it narrow

no-op feels like a safer first step - it doesn't change behavior for anyone already using prune. prompt + -force is a separate, bigger change, but i can rework it if that's the preferred direction

@jglogan what do you think?