.dockerignore, internal/build: Read git information directly from file (#15458)

* .dockerignore, internal/build: Read git information directly from file

This commit changes the way of retrieving git commit and branch for build
environment from running git command to reading git files directly.

This commit also adds required git files into Docker build context.

fixes: #15346

* .dockerignore: workaround for including some files in .git