feat(Dockerfile): switch cmd to entrypoint

By using `ENTRYPOINT` for the default command, it is possible to extend the command with arguments by using `CMD` in a custom Dockerfile. As the command itself doesn't need to be changed/overwritten with this approach, changes to the location of the binary or command will not break these configurations in the future.

Signed-off-by: Joda Stößer <git@simjo.st>

Author: SimJoSt

Dockerfile

@@ -22,4 +22,4 @@ FROM scratch
 COPY --from=build /home/rust/src/target/x86_64-unknown-linux-musl/release/notify_push /
 EXPOSE 7867
 
-CMD ["/notify_push"]
+ENTRYPOINT ["/notify_push"]