D058 - selfish image exists because another session asked for it in a script comment

obSCEne's scripts/build-pkg.sh was written by a concurrent session against a selfish that could not yet build an image, and it said exactly what it needed: "Add e.g. selfish image --root DIR --out IMG (or a directory mode on pack)". Both now exist.

Its call needed one correction that is worth recording, because it is the kind of mistake that produces two files which each look fine: image takes --content-id, and it has to be the same string the package declares. The image is encrypted under a key derived from it, so a mismatch yields a package whose filesystem cannot be opened by anything, with nothing visibly wrong in either half.