feat: allow commas in --image-label but not multiple labels

[LouisBrunner]

Resolves #622

Context

Currently, --image-label expects a list of key=value labels separated by commas. This means it is impossible to have labels whose value contain commas.

This is problematic for descriptions (as shown in #622) but also labels that mandate commas like moby.buildkit.frontend.caps (e.g. moby.buildkit.frontend.caps=moby.buildkit.frontend.inputs,moby.buildkit.frontend.contexts).

Those commas also can't be escaped to make pflag keep the values as one label.

Fix

TL;DR: this change might be considered a breaking change due to CLI flags not doing the exact same thing as before.

The issue can be solved easily by replacing StringSliceVar with StringArrayVar. However, it does mean --image-label foo=bar,baz=qux will stay as ["foo=bar,baz=qux"] instead of the current ["foo=bar", "baz=qux"] which might be unexpected for current users of --image-label.

From the name (--image-label) I did expect it to take only a single value instead of a comma-separated one but the description (Which labels (key=value) to add to the image.) seems to say the opposite.

Alternative

Trying to support both options in one flag (--image-label) seems difficult and quite error-prone so might it be possible to consider adding a new option (e.g. --image-add-label, --image-single-label, etc) which can support label with commas?

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