Add to the docs how to DRY on multiple crontab lines and use a shell script instead #100

When setting up multiple lines in crontab, repeating $(command -v docker) run --rm --env-file=/home/bob/.bitcoin-dca ghcr.io/jorijn/bitcoin-dca:latest [arguments...] is annoying and error prone.

Proposed solution
I'd recommend to add to the Tips section of the docs file cron examples how to create a shell script file to reference in the crontab and pass the arguments from the crontab to this shellscript.