It would not be equally safe because you are running the container on a machine that is connected to the internet. Even if the guest is not compromised, the host could be, and that would compromise the guest.
Furthermore, docker containers are not even fully fledged virtual machines. They share the host's kernel as well as a lot of other components from the host system.
In the end, the software that you run in a docker container gets executed by something in the host OS, and that has the potential to leak information to an attacker who has compromised the host.
Running Tails on an offline machine removes any existing OS as a factor since Tails is itself a fully fledged OS running on bare metal. You can ensure that the Tails you are running is that which is officially distributed, thereby ensuring that there isn't something else which has compromised it (of course there is the risk that Tails itself is compromised, but that's a different discussion). By physically disconnecting the device from the internet, there is no way for anything to leak to the internet.
