feat: Improve Docker image entry

Make `typos` the `ENTRYPOINT` rather than cmd, and output `--help` by
default.

Provides for better UX when running the image without arguments, and
facilitates just passing args to `typos` instead of having to repeat the
command name.

BREAKING CHANGE: `typos` must no longer given as the first argument
when running the Docker image, as it is the default `ENTRYPOINT` now.

Dockerfile

@@ -5,4 +5,5 @@ RUN cargo install --path .
 
 FROM debian:buster-slim
 COPY --from=builder /usr/local/cargo/bin/typos /usr/local/bin/typos
-CMD ["typos"]
+ENTRYPOINT ["typos"]
+CMD ["--help"]