DigestReference returns true if ref is a digest reference; i.e.

DockerInitPath figures out the path of our dockerinit (which may be SelfPath()).

ExperimentalBuild is a stub which always returns false for builds that do not include the "experimental" build tag.

GetCallerName introspects the call stack and returns the name of the function `depth` levels down in the stack.

GitClone clones a repository into a newly created directory which will be under "docker-build-git".

ImageReference combines `repo` and `ref` and returns a string representing the combination.

ReadDockerIgnore reads a .dockerignore file and returns the list of file patterns to ignore.

ReplaceOrAppendEnvValues returns the defaults with the overrides either replaced by env key or appended to the list.

SelfPath figures out the absolute path of our own binary (if it's still around).

TestDirectory creates a new temporary directory and returns its path.

ValidateContextDirectory checks if all the contents of the directory can be read and returns an error if some files can't be read symlinks which point to non-existing files don't trigger an error.