Changing configuration from windows' to linux syntaxis.
You need to rename each .exe in dosboxDK_single.conf according to actual names (not keeper.exe but KEEPER.exe) because linux is case sensitive, also replace ".." with "." (two dots to single dot).
Assuming you have dosbox from repository, create start.sh with the followind line, make it executable.
dosbox -conf "dosboxDK.conf" -conf "dosboxDK_single.conf"
Adjust cycles in former file until between being able to read after-mission statistic or scroll keymaps and easily slapping imps. On my pc it's about 120000 cycles. On cycles=auto game plays fine, but you can´t scroll -- too fast. Biggest slowdown seems to be amount of shadows per creature. Lower it to single. Don´t forget to press Alt+r once to change resolution.

Start the shell script and play with music (music volume adjustement works too). That's apply for every DOSbox game on GOG, even some linux versions might benefit from it like Shadow Warrior (1997).

Some advice:
#Sharpest output
output = openglnb
#More memory and it barks warning in terminal
memsize = 31
#Machine for less glitches in other games
machine = vesa_nolfb
#Sharpest, closest to original (tested on X-Com:UFO-defence) scaler, use 2x for 1080p, 3x for 2160p
scaler = normal3x