Keep secrets out of the Docker build context at every depth (closes #205)

.dockerignore patterns without a leading **/ match only at the root of
the build context, so a nested .env or private key still reached it and,
through COPY . ., a build-stage layer. The file is now the standard one
from sneak/prompts: every pattern that should match anywhere has **/,
and private keys and environment files are matched in any letter case.

pixa keeps its own differences: .git is still sent without .git/config
in place of the standard .git line, which the version stamp needs, and
.gitignore, /bin and /data stay out.

Model: opus-5-5

TODO.md

@@ -31,6 +31,12 @@ P2: security: per-IP rate limiting on the image routes
 
 # Completed Steps
 
+- 2026-10-04 `.dockerignore` keeps secrets out at every depth (closes #205): the
+  file is now the standard one from `sneak/prompts`, whose patterns match in
+  every directory and, for environment files and private keys, in any letter
+  case, so a nested `.env` or `server.key` no longer reaches the build context.
+  pixa still sends `.git` without `.git/config` in place of the standard file's
+  `.git` line, and still leaves out `.gitignore`, `/bin` and `/data`.
 - 2026-10-04 an integration test of the image proxy flow (closes #80):
   `TestImageProxyFlow` in `internal/server` starts the database, handlers and
   middleware from the constructors `pixad` uses, with a fresh state directory,