https://gitlab.mister-muffin.de/josch/img2pdf
As mentioned by PleaseStand, GhostScript (edit: prior to v9.23) will decode the JPEG data, resulting in generation loss, as well as performance “ten to hundred” times worse than img2pdf.
Edit: Ghostscript 9.23 (2018-03-21) added a “JPEG Passthrough” capability that resolves the generation-loss/performance issue.
ImageMagick (i.e. convert) also decodes and re-encodes the images.