Install dotnet-sdk-8 instead of 9 in Dockerfile

[coolreader18]

Description of Changes

spacetime init generates a global.json with dotnet 8, so install 8 instead of 9 in the image. Also rm /var/lib/apt/lists/*, which is what apt update downloads, since that's unnecessary to package when the user can just run apt update themselves if they really need to.

Expected complexity level and risk

1

Testing

The rust module generated by spacetime init works (including publish, call, logs, etc), since I was able to use a git dependency because rc4 isn't published to crates.io yet, but I couldn't figure out how to use a git dependency in dotnet.

The invocations for using the image are:

# start it up, using the current directory as the module to publish
docker run --init --name mydb -v $PWD:/app spacetime start
docker exec -ti mydb spacetime <init,publish,call,logs,etc>