docker: consider each requirement when updating dependencies

[mctofu]

This change works toward resolving #3173 with an alternate approach to #3251. It maintains grouping of updates so that we can update php:8.0.1-apache and php:8.0.1-cli in a single PR instead of two as suggested in #3251 (review).

Prior to this change if there were multiple tags with different prefixes/suffixes we'd only resolve the latest version for one of them and would end up updating all of the tags to a common version:

=== php (8.0.1-apache)
 => checking for updates 1/1
 => latest available version is 8.0.3-apache
 => latest allowed version is 8.0.3-apache
 => requirements to unlock: own
 => requirements update strategy: 
 => updating php from 8.0.1-apache to 8.0.3-apache

    ± Dockerfile
    ~~~
    1c1
    < FROM php:8.0.1-apache
    ---
    > FROM php:8.0.3-apache
    ~~~

    ± Dockerfile.cli
    ~~~
    1c1
    < FROM php:8.0.1-cli
    ---
    > FROM php:8.0.3-apache
    ~~~

This change resolves the latest version for each tag we can correctly preserve the prefix/suffix in the update:

=== php (8.0.1-apache)
 => checking for updates 1/1
 => latest available version is 8.0.3-apache
 => latest allowed version is 8.0.3-apache
 => requirements to unlock: own
 => requirements update strategy: 
 => updating php from 8.0.1-apache to 8.0.3-apache

    ± Dockerfile
    ~~~
    1c1
    < FROM php:8.0.1-apache
    ---
    > FROM php:8.0.3-apache
    ~~~

    ± Dockerfile.cli
    ~~~
    1c1
    < FROM php:8.0.1-cli
    ---
    > FROM php:8.0.3-cli
    ~~~

One caveat is this only works if the tags are in different files. If a Dockerfile includes two tags with different prefixes/suffixes then we still end up replacing both of them with a single tag. Fixing that requires splitting out to multiple dependencies and updating the file updater to handle multiple dependencies. I started going down that road (b5b80df) but that's going to be a bigger chunk of work and I think it would be better to release this initial fix first.

[feelepxyz]

Thanks for splitting this out of the file updater WIP, shipping this first makes lots of sense to me 👍

Changes LGTM 💯