By default, in order to increase performance during IO operations, Ruby
doesn't immediately write to the standard output but uses a buffer
internally and writes the output in chunks [1].
It looks like this results in some output being missed when running
Docker Compose [2], so we're activating the sync mode, which flushes all
output immediately.
[1] https://ruby-doc.org/core-2.6.5/IO.html#method-i-sync-3D
[2] See issue 1118 in the sinatra/sinatra repository
fa56a74ba0