cmd, internal/build, docker: advertise commit date in unstable build versions (#19522)

* add-date-to unstable

* fields-insteadof-split

* internal/build: support building with missing git

* docker: add git history back to support commit date in version

* internal/build: use PR commits hashes for PR builds

.dockerignore

@@ -1,7 +1,3 @@
-**/.git
-.git
-!.git/HEAD
-!.git/refs/heads
 **/*_test.go
 
 build/_workspace