"Docker run" within merge-two-sides.sh outputs error in crontab

[groschenopa]

Hi Marty,
when I use your script in itself, everything works great. But when I use it within crontab like you instructed in the comments, the logfile only gives me this:

/share/CACHEDEV1_DATA/Public/Dokumente/Inbox/merge-two-siders.sh: line 25: docker: command not found

Within the .sh, the line looks like this for me:

docker run --rm -v $DIR:/app -w /app minidocks/ghostscript -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite -sOutputFile=${OUTPUT_FILE} ${FILES}

What seems to be the problem here?
I'd really like to use your great script, but it stands and falls with its ability to work automated.

Thank you so much!