Suggest a .dockerignore file if none exists

[jordanstephens]

If no dockerignore file exists, prompt the user to create a default one. That default one should include everything listed here:

defang/src/pkg/cli/compose/context.go

Line 44 in 27f5599

defaultDockerIgnore = `# Default .dockerignore file for Defang

We are currently (silently) inserting a default .dockerignore, but I am concerned about making too silent many assumptions about whether or not a user expects these files to be present in the final build. Ultimately, I think creating the file for them to edit is an excellent compromise.