Trim the build context and drop unused imports

Add a .dockerignore that whitelists only requirements.txt, start.py and
dsc/. The build context previously included .git, .idea, __pycache__ and
any local .env, which slowed builds and put a secrets file within reach
of any future COPY of the whole tree.

Remove the unused urlparse import from dsc/client.py and the redundant
import os.path from start.py; importing os already binds os.path.

Record both in IMPROVEMENTS.md, including the part still outstanding:
binutils and squashfs-tools remain in the runtime image because the
AppImage is unpacked in the same stage.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

start.py

@@ -3,7 +3,6 @@
 import argparse
 import logging
 import os
-import os.path
 import sys
 
 from dsc import SeafileClient, const