bug: building with go get results in invalid binary

[dalehamel]

Building as per our docs https://github.com/iovisor/kubectl-trace#source when building with go get, the resulting binary behaves bizarrely. This is mostly due to the lack of LDFLAGS being specified, and any variable which requires git to not be populated.

The outcome of this is that the images will have no tag, so will fail to run, and the version subcommand will be empty.

To fix this, we should ensure that these variables have reasonable defaults for when they are NOT specified, for instance, we can default the version to master, and the image tag to latest, as well as ensure we are pushing the :latest tag on pushes to master.

[dalehamel]

@leodido @fntlnz FYI

[leodido]

As discussed internally, I confirm those findings.

Also, I agree on the solution.

[dalehamel]

It looks like we SHOULD be already pushing :latest, but aren't:

https://github.com/iovisor/kubectl-trace/blob/master/Makefile#L117-L121

https://github.com/iovisor/kubectl-trace/blob/master/build/scripts/ci-release-image.sh#L28-L30

I think this is because github changed how github.ref works on github actions IIRC, so it may be that it never finds master. I think I ran into this somewhere else as well. Should be an easy fix.

[dalehamel]

https://github.com/iovisor/kubectl-trace/blob/master/Makefile#L36 we'll need to do a pass over all these variables and ensure their defaults are sane