Enable backtracing for images created with the plugin

[heckj]

Include swift-backtrace-static, from the swift static SDK, alongside the binary when it's copied into the container in order to allow the container to capture stack traces on a crash and report them.

I'm tempted to ask for a way to set a default environment variable (SWIFT_BACKTRACE) on the container as well, but the permutations get large for the docs of that feature, and that can be set at runtime as well, so I'm not certain it's nearly as critical as just enabling the binary to capture the stack traces.

[Joannis]

This would be very helpful

[euanh]

@Joannis Yes, this would definitely be a nice feature to have. We would need to include swift-backtrace-static and set the SWIFT_BACKTRACE environment variable:

• The --resources flag adds arbitrary extra files to the container image, so could be used to inject swift-backtrace-static.
• We don't currently have support for setting the container's default environment variables, but it should not be too difficult to add: containertool should be able to set or override a container image's environment variables #86

To do this automatically, the plugin would need to know where to find an appropriate swift-backtrace-static binary. In principle it could fetch it from the SDK, although I'm not entirely sure that a SwiftPM plugin can see the path to the SDK being used for the build.

For now, the easiest way to support backtraces is to build a custom "distroless-style" base image, including just swift-backtrace-static and maybe the default TLS trust roots and other usual distroless things. You would then use the plugin to stack your service binary on top of this base image using the --from argument. That should still result in a relative small overall image.

[Joannis]

@euanh for the lack of a better channel to contact you: will you be at server side swift conf? I have something to discuss related to this plugin I'd like to take offline or into a call. I could probably work on this issue and a couple others if we have alignment.

[euanh]

@Joannis Yes, I'll be there.