Improve support for bootstrapping a development environment without an active network connection

[freakboy3742]

When you run a briefcase command for the first time, it requires an active network connection, and will download multiple gigabytes of tools. This is particularly bad on macOS/iOS (which require Xcode) and Android (which needs the sdkmanager an associated tools). This is a problem in bandwidth-limited scenarios (like tutorial sessions or conference wifi). It can also be a problem in environments where network access is limited or compromised (e.g., behind aggressive corporate firewalls)

It should be possible to give someone a USB drive populated with files, and bootstrap their briefcase installation.

Describe the solution you'd like

A new command:
briefcase bootstrap <platform> <folder>

will inspect <folder>, and install all the tools needed for <platform>. This may not 100% remove the need for network activity, but it should be possible to dramatically lower the initial bandwidth/time requirement.

A parallel command/option will also be required to create the archive folder in the first place (briefcase bootstrap --create <platform> <folder>?)

Describe alternatives you've considered

No real options, other than requiring an active network connection.

Additional context

Support packages and templates can be copied directly from the source folder; it may be desirable to require them to be stored in zip format.

The Android sdkmanager has support for a --package_file argument that can be used to directly install packages from a zip file. It's not clear

There is likely not much we can do about Xcode, as it can't be distributed outside the App Store.

[jlgimeno]

It is possible to download and install the Xcode .xip file if one has an Apple Developer account. I'd say that would be an edge case for the typical briefcase user.

[freakboy3742]

See #388 for an other issue related to offline operation

[mhsmith]

To build offline for Android, as well as sdkmanager packages, you would also need cached copies of all the necessary Gradle plugins and libraries. I think the only feasible way of doing that would be to actually do a build, and then save a copy of the ~/.gradle directory.

[freakboy3742]

The code is open source, so you're of course free to fork if you choose... but is there a reason you've chosen to advertise a fork, rather than contribute a pull request?

I'd be more than happy to review a PR implementing an offline mode, and merge that change if/when it passes review - that's why this is an open feature request on the repo.

[freakboy3742]

For posterity - the fork described here is a non-starter from the perspective of Briefcase as a project. It addresses a very limited subset of bootstrapping, in a way that isn't compatible with the long-term maintenance of Briefcase templates. See the review of the PR for details.