add template funcs

[coryb]

This adds the sprig template funcs for general usage
Also I have added several docker image parsing template funcs which I have needed for various docker image publishing routines.

[slushie]

Does it have to be dockerFoo? Could this be imageRefDomain or something a bit more OCI-friendly? 😉

[hinshun]

Was chatting on slack, but I was suggesting since we had interpolation support in heredocs, I prefer if we use regular string functions rather than template functions.

Along with the array support proposal, you could do something like:

[]string dockerRegistries() {
	"titusregistry.us-east-1.netflix.net"
	"titusregistry.us-west-2.netflix.net"
}

[]string pushTargets(string ref) {
	for (string registry in dockerRegistries) {
		"${registry}/${dockerPath(ref)}:${dockerTag(ref)}
	}
}

fs titusPush(string ref) {
	for (string target in pushTargets(ref)) {
		dockerPush target
	}
}