Keep git-ignored files and data/ out of the Docker build context (closes #266)

.dockerignore now lists every .gitignore pattern, each with **/ so Docker
matches it in every directory as git does, plus the top-level data/
directory. git-ignored secrets such as .env.local, *.key files and
data/session.key no longer reach the build stages or the build cache. .git
stays in the context and no tracked file is listed, so the version still
comes from git describe without -dirty.

data/, where upaasd keeps its database and session key when run from the
checkout, is now git-ignored.

Model: opus-5-5

TODO.md

@@ -20,6 +20,11 @@ regress.
 
 # Completed Steps
 
+- 2026-10-02: `docker build .` no longer sends git-ignored files, such as
+  `.env.local`, `*.key` files or upaasd's `data/` directory with its session
+  key, into the build stages and the build cache: `.dockerignore` now leaves out
+  everything `.gitignore` does, and `data/` is git-ignored (#266).
+
 - 2026-10-02: The build no longer passes the CPU architecture in: upaas reads it
   from Go's `runtime.GOARCH` when it runs, and the startup log line reports it
   as `arch`. `CONVENTIONS.md` follows the updated conventions in `sneak/prompts`