compose file: use v3 and no volume

The current target didn't have jekyll installed, so mounting the
source-files didn't do anything.

This patch removes the bind-mount.

Alternatively, we could switch back to using a jekyll stage (with
automatic rebuilding).

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docker-compose.yml

@@ -1,4 +1,4 @@
-version: "2"
+version: "3.7"
 services:
   docs:
     build:
@@ -8,5 +8,3 @@ services:
     image: docs/docstage
     ports:
      - "4000:4000"
-    volumes:
-     - "./:/usr/src/app"