Fix the PHP 7.2 build on Debian 10.

[johnbillion]

As seen in the workflow runs of #172 the PHP 7.2 build is failing because the Debian 10 packages have moved to the archive. Previously: #106.

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[desrosj]

I was looking at whether we could remove 7.2 from the list of containers that are regularly rebuilt. Now that Debian 10 is EOL, I think the only remaining dependencies still receiving updates are PHPUnit 8.x and memcached.

I looked into which versions of PHPUnit were used by various branches of Core. It seems that even though versions 6.x through 8.x were supported for various branches, they all seem to run 7.x. That needs to be investigated further to make sure it's intentional and correct.

For now, this should definitely be fixed. 👍