optimize some containers

[kklepper]

Is your feature request related to a problem? Please describe.
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Some containers seem to be rather fat. I noticed

jeboehm/mailserver-db latest bb63c6658001 11 days ago 448MB

Compare this to

kklepper/mariadb33 alpine ec5eb0ced11b 5 months ago 112MB

Also

jeboehm/mailserver-web latest 34ef8421291a 11 days ago 430MB

and

kklepper/nginx-php7-mysqli-memcached alpine 53748deefdef 4 weeks ago 35.2MB

Describe the solution you'd like
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I think it would be worthwhile to slim down these two.

Describe alternatives you've considered
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I did not try to replace yours with mine -- there are too many imponderables.

Additional context
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I could inspect your dockerfiles or provide mine for you to inspect.