Docker performs only container virtualization, meaning all docker containers are running on the Linux kernel of the host system, just isolated using Linux cgroups. So if your host system is ARM-based it can only run ARM-based containers or generally speaking: Docker can only run containers build for the same architecture as the host system.
So Docker (and other container virtualization systems) do not help with this issue. What you need is full hardware virtualization . A popular software of this category is QEMU which can for example emulate a x86 system on an ARM-based Linux. It should be able to run on Android too. But be aware that this full virtualization is very expensive in terms of resources and adds a lot of overhead to the software running inside of it. So you probably won't get the performance from MongoDB you would expect, especially on relatively low-end processors like the ones in smartphones.
The only real solution is to add native ARM v8 support to MongoDB itself. More and more servers out there are using ARM architecture, so this is quite inevitable.