Make clockworklabs/spacetime docker image build from source

[coolreader18]

Description of Changes

Fixes #2509, #2511.
Motivation from the latter:

Should probably just build from source. Otherwise you could get into a bad situation where a user has clockworklabs/spacetime:1.0.1 but has updated the cli inside to 1.2.0 and is confused why they're running into issues.

The dotnet install script is kinda flaky right now, see dotnet/install-scripts#582. If it's not building correctly replace https://dot.net/v1/dotnet-install.sh with https://builds.dotnet.microsoft.com/dotnet/scripts/v1/dotnet-install.sh and it should be exactly equivalent.

Expected complexity level and risk

1

Testing

• Tested that spacetime works and cargo exists as a command.

[jdetter]

This seems to work, thanks @coolreader18 ! Tested via:

docker build -t spacetime-test .
docker run --rm -p 3000:3000 spacetime-test start

Then I logged in via

docker exec -it <name> /bin/bash

And I was able to spacetime normally, including publishing to the local server.