Can you provide a minimized test case (short single C source file) to
reproduce this, or an strace log of the test that fails? The latter is
probably actually be better if the behavior is dependent on the Docker
network configuration. Assuming the test is attempting to lookup and
bind on "localhost" by name, which is what it appears to be doing
here:

https://github.com/apache/thrift/blob/82ae9575cdc112088771fc7b876f75e1e4d85ebb/lib/cpp/test/TServerSocketTest.cpp

the behavior you're experiencing is not what I expect from musl;
rather my expectation is that you would get 127.0.0.1 as the first
result and ::1 as the second, and this is exactly what I see if I do:

ip addr del ::1 dev lo

on my laptop running Alpine, then call getaddrinfo for localhost with
a small test program.

The logic to sort results does gratuitously depend on v4mapped
addresses working to do the IPv4 routability probing; if something
about the configuration suppresses their ability to work, it will
break. This is a known open issue I want to go back and fix. Seeing
the strace would show me right away if it's the source of your
problem, and even if not chances are very good that it would point out
whatever the cause is.