Switch to using docker for tests

Also bump minimum version to 3.12 since that's the oldest I can test
4 jobs for phpunit-5.7 in 1 minute and 26 seconds (queued for 1 second)
Status Job ID Name Coverage
  Typecheck
failed #741
docker
hack

00:00:39

failed #740
docker
hack-min

00:00:47

 
  Test
skipped #743
docker
phpunit
skipped #742
docker
phpunit-min
 
Name Stage Failure
failed
hack-min Typecheck There is an unknown failure, please try again
Cloning into '/builds/91-carriage/phpunit-hhi'...
Checking out e421e558 as phpunit-5.7...
Skipping Git submodules setup
Checking cache for hack-min/phpunit-5.7...
Successfully extracted cache
$ echo 'assume_php=false' > .hhconfig
$ composer install
/bin/bash: line 47: composer: command not found
ERROR: Build failed: exit code 1
failed
hack Typecheck There is an unknown failure, please try again
Cloning into '/builds/91-carriage/phpunit-hhi'...
Checking out e421e558 as phpunit-5.7...
Skipping Git submodules setup
Checking cache for hack/phpunit-5.7...
Successfully extracted cache
$ echo 'assume_php=false' > .hhconfig
$ composer install
/bin/bash: line 47: composer: command not found
ERROR: Build failed: exit code 1