Fix docker container build

[cliffhall]

Describe the bug
When we tried #257, it failed with:

ERROR: Multi-platform build is not supported for the docker driver.
Switch to a different driver, or turn on the containerd image store, and try again.

What driver are we using?

In CI, the setup for docker/build-push-action says it's using docker as the driver.

What driver should we be using?

According to the Docker docs, the driver should be: docker-container instead of docker

The usage section of the docker/build-push-action seems to indicate that setup-buildx-action (which we are using) will do that, though:

setup-buildx action will create and boot a builder using by default the docker-container driver. This is not required but recommended using it to be able to build multi-platform images, export cache, etc.

To Reproduce
Steps to reproduce the behavior:
CI failure

Expected behavior
Should build and push the container to the ghcr registry

Logs

Additional context
See #257

[cliffhall]

LOL, was looking at a failure in @aaronpowell's repo somehow.