Add phpdbg to zts images

[dercoder]

Regarding to the issue #1260 I would suggest to add the phpdbg again to ZTS images.
It would be better to add it for all images again but ZTS is at least no FPM image

The workarround

COPY --from=php-cli /usr/local/bin/phpdbg /usr/local/bin/

will also not help in this case

[tianon]

Sorry for the delay! This is an interesting point -- the ZTS images are spiritually the same as the CLI images, but with ZTS enabled, so I think it probably does make sense to re-enable phpdbg there.