Docker's behaviour with a relative ENTRYPOINT?

Dockerfile

FROM busybox

ENTRYPOINT ["echo", "Hello", "World"]

Q: How does docker (Docker) behave with a relative ENTRYPOINT?

See:

$ docker build -t foo . && docker run -i -t --rm foo
[+] Building 2.5s (6/6) FINISHED
 => [internal] load build definition from Dockerfile                                                                                                       0.0s
 => => transferring dockerfile: 94B                                                                                                                        0.0s
 => [internal] load .dockerignore                                                                                                                          0.0s
 => => transferring context: 2B                                                                                                                            0.0s
 => [internal] load metadata for docker.io/library/busybox:latest                                                                                          2.4s
 => [auth] library/busybox:pull token for registry-1.docker.io                                                                                             0.0s
 => CACHED [1/1] FROM docker.io/library/busybox@sha256:ad9bd57a3a57cc95515c537b89aaa69d83a6df54c4050fcf2b41ad367bec0cd5                                    0.0s
 => exporting to image                                                                                                                                     0.0s
 => => exporting layers                                                                                                                                    0.0s
 => => writing image sha256:697494dc104cf1b1b551f623d35e16b48c1d649aaf7ec01c911cfe9815e516d8                                                               0.0s
 => => naming to docker.io/library/foo                                                                                                                     0.0s
Hello World

Q: How does this end up running in a shell?! Which shell?!