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>

IMPROVEMENTS.md

@@ -171,10 +171,14 @@ Acceptance criteria:
 
 - [ ] Separate configuration, subprocess execution, library resolution, and
   status monitoring so each can be tested independently.
-- [ ] Remove unused imports and unnecessary dependencies.
+- [x] Remove unused imports and unnecessary dependencies. Both pinned
+  requirements are used; `urllib3` is imported directly but only pinned
+  transitively through `requests`.
 - [ ] Add type annotations and static checks at external-input boundaries.
 - [ ] Make logging configurable and avoid forced dependency debug logs.
-- [ ] Add `.dockerignore` and reduce unnecessary image packages.
+- [ ] Add `.dockerignore` and reduce unnecessary image packages. `.dockerignore`
+  is in place; `binutils` and `squashfs-tools` are still shipped in the final
+  image because the AppImage is unpacked in the same stage.
 
 ### Establish a visible maintenance process