Add entrypoint and working-dir in the base image

[rambip]

It would be nice to add ´.cargo/bin/trunk´ as an entrypoint in your dockerfile, even if it is overridden by the dockerfile that uses it.

And also adding ´mkdir /web´ and ´WORKDIR /web´ to the Dockerfile.

That way people would be able to do ´podman run -v ./:/web:z osomahe/rust-trunk build´ in a single command if they want to.

[rambip]

and also ENV RUSTUP_HOME=/root/.rustup