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

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

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

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

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

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

Figure 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.