Base docker image isn't signed

[scottcha]

I was testing this out and noticed that the base docker image referenced isn't signed. This causes the steps in the Readme to fail unless you either download that image with docker pull first or if you do the docker build telling it not to check signing.

Expected result:
Can run docker build and it succeeds on clean device.

Actual result:
Docker build command fails when checking signing.

[scottcha]

I'll provide a PR you can review if you are interested which moves the docker base image to the signed pytorch image and simplifies the docker build quite a bit.