chore(docker): ignore worktrees/git/node_modules from build context

Every docker build sent the whole repo root as context, including every git
worktree under .claude/worktrees/ (200-300MB each) — that crossed 390MB of
cruft and starved mac-mini's disk mid-build on 2026-07-27 (873b00a). None of
it belongs in an image.

.dockerignore

@@ -0,0 +1,12 @@
+# Every docker build in this repo previously sent the whole directory as
+# build context — including every OTHER git worktree under .claude/worktrees/
+# (each with its own web/node_modules, ~200-300MB apiece). That's what
+# starved the mac-mini's disk mid-build on 2026-07-27 (SHA 873b00a): the
+# context alone crossed 390MB of pure worktree cruft before the host ran out
+# of space. None of this ever belonged in an image.
+.claude/worktrees/
+.git/
+**/node_modules/
+**/dist/
+**/build/
+*.log