Add docker build.

[Tim-Nosco]

This Dockerfile builds qemu after applying the patch, then builds cannoli in a separate stage.

This container will make a CI release possible in a future patch.

[Tim-Nosco]

To test the docker feature:

First, build with docker build -t cannoli .

Then in one terminal, run cannoli:

docker run \
    -it \
    --rm \
    -w /build/cannoli/examples/symbolizer \
    cannoli \
    /root/.cargo/bin/cargo run --release

And in the other, run the application:

docker exec \
    -it \
    -w /build/cannoli/examples/symbolizer \
    $(docker ps -q) \
    bash -c " \
        /build/qemu/qemu-mipsel-patch \
            -cannoli /build/cannoli/target/release/libjitter_always.so \
            ./example_app && true \
        "