play-with-docker: qemu-user not detecting binary format

The test script in 1138-4EB/test-qemu statically cross-compiles a Hello World C program inside a ubuntu:18.04 container and then executes it in a different container with qemu-user. It works as expected in Ubuntu and Fedora hosts (x86_64), and produces the following last two lines:

Hello world!
Hello world!

However, in play-with-docker, this is the result:

bash: ./main: cannot execute binary file: Exec format error
Hello world!

The difference between the two calls to the executable is:

./main; \
qemu-aarch64 ./main

I’d like prepending qemu-aarch64 not to be required, because this is just a stripped down version of a problem produced when trying to cross-compile libelf (where adding it is not so straighforward).

Is there any additional tweak or argument required in play-with-docker for it work?

Just in case it might be relevant, the same error occurs in travis-ci: https://travis-ci.com/1138-4EB/test-qemu/builds/77530556