Stop shipping the sprite pipeline

assets/_sheet_*, assets/_pose_ref_* and the generator scripts are what the
frames were CUT OUT OF, not what the game loads — 34 of the 45 MB in
assets/, in the image and publicly served, fetched by nobody. A
.dockerignore on the underscore prefix takes assets/ to 16 MB, with
nothing to maintain per sheet: the SPR list in render.js has no
underscore-prefixed name in it.

Also keeps .git (83 MB) out of the build context.

Not verified against a real build — no docker daemon on this machine.

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

.dockerignore

@@ -0,0 +1,12 @@
+# Everything under assets/ whose name starts with _ is the SPRITE PIPELINE, not the game: the
+# generator scripts, the reference poses, and the big source sheets _slice.py cuts the frames out
+# of. That is 34 MB of the 45 MB in assets/, no client ever fetches one, and until now they were
+# all sitting in the image and publicly served. The game loads no underscore-prefixed file — see
+# the SPR list in render.js — so the rule is the prefix, with nothing to maintain per sheet.
+assets/_*
+
+.git
+.env
+HANDOFF.md
+**/__pycache__
+**/.DS_Store