set WORKDIR for final image, improve help text

gamedl

@@ -20,6 +20,7 @@ RUN \
         wget -O trovefix.patch 'https://patch-diff.githubusercontent.com/raw/xtream1101/humblebundle-downloader/pull/59.patch' &&\
         git apply trovefix.patch &&\
         pip install -e .
+WORKDIR /dl
 CMD echo "Usage: gamedl <hbd|itch-download|help|build> [<options>]"
 EOF
 }
@@ -46,11 +47,23 @@ function run_image () {
 
 function show_help () {
 	echo "Usage $0 <command> [<options>]
-Commands:
+
+DESCRIPTION
+	Run game platform downloaders in a docker container. Currently supported are:
+	- Humble (https://pypi.org/project/humblebundle-downloader/)
+	- Itch.io (https://pypi.org/project/itchiodl/)
+
+	The downloaders run in a docker container with your current working dir
+	mounted to /dl inside the container.
+
+COMMANDS
+	Every command except help and build get forwarded into the container,
+	so you can run arbitrary commands if you know what you are doing.
+
 	hbd
-		Run the Humble downloader.
+		Run the Humble downloader. Run 'gamedl hbd -h' for help.
 	itch-download
-		Run the Itch.io downloader.
+		Run the Itch.io downloader. Run 'gamedl itch-download -h' for help.
 
 	build
 		(Re)build the gamedl-image.