From internal bug tracker:

"When configuring an out of tree build using jib, the --src-dir option is required to tell jib where to find the source tree. Failure to do so leads to a really obscure error that provides no clues as to what's gone wrong or what to do about it. For most uses, it would be convenient if the --src-dir defaulted to the location of the invoked jib script."

The information about where the jib script is located needs to be captured by the script in the source. I think the best way of propagating that information to Jib itself is through the environment variable JIB_SRC_DIR, which can already be set to provide a default value for --src-dir. The launcher script can set this variable if it's empty.