Thanks guys, I had the same problem and found the fix here. It is indeed issueing the "c:" command before the mount command.
Just being curious: Did anyone figure out why this matters? Why is it wrong mounting the CD image before switching to c?
Actually Wild_Penquin wrote the reasons why it fails but here are my 2 cents.
Mounting fails because of the different path separator on linux. The GOG Dosbox configuration is for windows filesystems (case insensitive)
Dosbox tries to load the image from your local filesystem (case sensitive, / as path separator) and the dos environment (case insensitive, \ as path separator)
Before switching to c: dosbox tries to mount the image from the local file system and from Z: and fails on local fs because of the wrong path separator and Z: because the path does not exists.
It works after the switch to C: because the path inside the dos env is correct now.
For Arch Linux you can use this AUR package and have look at the needed changes for linux. https://aur.archlinux.org/packages/battle-isle2-gog