Refactor Dockerfile to build Go binary outside of Docker (#218)

This change moves building the Go binary out of the Dockerfile, and also
fixes some issues in the makefile. These things will speed up build
times and avoid rebuilding when not needed. Additionally it will make
getting version data easier to use as part of the user agent.

Signed-off-by: Philip Laine <philip.laine@gmail.com>

.dockerignore

@@ -1,3 +1,2 @@
-# More info: https://docs.docker.com/engine/reference/builder/#dockerignore-file
-# Ignore build and test binaries.
-bin/
+**
+!bin/*/netbird-operator