!override can be used to replace value

[ndeloof]

Introduce !override so one can replace any entry in compose.override.yaml, while !reset would remove but not allow to re-define
i.e:

• foo: !reset null removes foo attribute from merged yaml (value is ignored)
• foo: !override hello overrides any previous foo attribute value with new value "hello"

TODO: document this on the compose spec

fixes docker/compose#11408

[glours]

LGTM

[laurazard]

This makes it read a bit weird, no? If !reset only removes and does not allow to redefine, having to write:

foo: !reset null

is a bit weird, I would expect to just write

foo: !reset

Can we assume null when there is no value for !reset? (ignore this if that's already something we can do)

[ndeloof]

@laurazard I we one can, but !reset is just a yaml tag, yaml syntax requires a value. Can also use:

foo: !reset {}

[laurazard]

RIght, I had a vague idea from when I was looking into the tags there was some reason why it was required. Regardless, that's unrelated to this change, !override sounds good to me.