Use computeIfAbsent to avoid creating unused objects

[csokol]

Small optimization using computeIfAbsent

[palantirtech]

Thanks for your interest in palantir/docker-compose-rule, @csokol! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.