Improve usability for garage in container by setting entrypoint (#1363)

- BREAKING: This update will probably break previous containers setups
that expect you to provide `/garage`

After the upgrade, instead of:
    docker run --rm dxflrs/garage:latest /garage --help
you need to run
    docker run --rm dxflrs/garage:latest --help

Signed-off-by: ieugen <eugen@ieugen.ro>

Reviewed-on: https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/1363
Reviewed-by: Alex <lx@deuxfleurs.fr>
Co-authored-by: ieugen <eugen@ieugen.ro>
Co-committed-by: ieugen <eugen@ieugen.ro>

Dockerfile

@@ -4,4 +4,6 @@ ENV RUST_BACKTRACE=1
 ENV RUST_LOG=garage=info
 
 COPY result/bin/garage /
-CMD [ "/garage", "server"]
+
+ENTRYPOINT ["/garage"]
+CMD ["server"]