Consider switching from CMD to ENTRYPOINT in docker image

[mccutchen]

As proposed by @x70b1 in #84, updating the Dockerfile to set the go-httpbin binary as the ENTRYPOINT rather than the CMD

- CMD ["/bin/go-httpbin"]
+ ENTRYPOINT ["/bin/go-httpbin"]

would be a small, useful UX improvement by allowing folks who run the image using command line arguments to slightly simplify their workflows:

-  nerdctl run mccutchen/go-httpbin go-httpbin -use-real-hostname
+  nerdctl run mccutchen/go-httpbin -use-real-hostname

I'm currently a little reluctant to do this out of fear of unexpectedly breaking existing uses in CI processes if they're not pinning to a specific release, but it's a good candidate to include in any future breaking change we need to make.