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 https://github.com/dotnet/install-scripts/issues/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

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