Advanced image metadata

[denzp]

Dockerfile statements:

EXPOSE 80/tcp
LABEL version="1.0"
VOLUME /data
STOPSIGNAL SIGKILL

Should have equivalent cargo-wharf metadata:

[package.metadata.wharf.output]
expose = ["80/tcp"]
labels = { version = "1.0" }
volumes = ["/data"]
stop_signal = "SIGKILL"