smp-server-download.Dockerfile does not verify smp-server binary

I just recently rebuilt the docker container running my smp-server, and encountered a failure mode when building the -download docker image. If github throttles you or there's some other network error when you attempt to download, the result is that your smp-server binary might just be a bad server response in a text file 😄

# cat /usr/bin/smp-server
<?xml version="1.0" encoding="utf-8"?><Error><Code>ServerBusy</Code><Message>Egress is over the account limit.
RequestId:f4b4d993-b01e-006c-0778-d3c9f2000000
Time:2022-09-28T20:26:34.2999215Z</Message></Error>

The result is the rather obtuse log message when the entrypoint tries to execute the bad text file:

/usr/bin/smp-server: 1: Syntax error: redirection unexpected

One solution to this might be publishing a shasum of the smp-server binary as an asset on your github releases, and then comparing that hash with the downloaded binary in the -download.Dockerfile build to ensure that your downloaded binary matches what you expect.

Start with smp-server-download.Dockerfile and the step that downloads /usr/bin/smp-server. Check how a failed or non-binary response is handled and whether the downloaded artifact can be compared with a release checksum. Done means the image build rejects an invalid download instead of producing an executable text file.