fix dockerignore and tarball uploads for cloud agents

[real-danm]

• loads the existing dockerignore if there is one
• uses the project type specific dockerignore if one isn't found
• add tests to ensure both top-level and subdirs do not include ignored files

adding a test workflow to run on windows, ubuntu, and mac

[bcherry]

Code looks solid and less brittle - do you have a windows machine available to test on?

[real-danm]

Code looks solid and less brittle - do you have a windows machine available to test on?

i don't have a windows machine set up to test this. i added a test run to the GH actions including windows. these new changes pass although there is still some windows issue we need to fix, will address that in a follow-up pr