add ability to override ints in compose files with 0

[mspiess]

fixes #3293

- What I did Allow the number for replicas to be overridden with 0 when using multiple compose files.

- How I did it I added a custom merge function for the type *uint64. Maybe there is a more elegant way, but I was struggling to find documentation or examples for the used library "mergo". This issue of the library might be related.

- How to verify it See the reproduction steps in the linked issue.

- Description for the changelog Fix overriding ints with 0 when using multiple compose files

:whale: Signed-off-by: Marco Spiess [email protected]