1 Environment Description
Operation System : macOS majove version 10.14.1
Docker created by: "docker-machine" and "docker" CLI, NOT "docker for Mac" app.

2 Reproducing steps in v18.09.0

1. Create a docker machine "tryIt", command is:
   docker-machine create tryIt --virtualbox-cpu-count "2" --virtualbox-memory "4096" --virtualbox-disk-size "100000"
   In this step, hit problem v18.09.0 does not embed version number in ISO header #1347, just workaround it. And successfully created.

2. Create a common ubuntu container, now the situation is:
   macOS------vboxnet1------192.168.99.1
   tryIt---------docker0-------172.17.0.1
   tryIt---------eth1-----------192.168.99.100
   container----eth0----------172.17.0.2

3. Try to ping container(172.17.0.2) from macOS(192.168.99.1) with adding a route
   Failed here in v18.09.0
   Ping successfully in v18.06.1-ce with exactly the same above steps

3 Other information
With command cat /etc/issue, found that:
v18.06.1-ce is based on "Core Linux"
v18.09.0 is based on "TinyCoreLinux 8.2.1"