Add a .dockerignore (closes #39)

Adds a .dockerignore, which REPO_POLICIES.md lists among the files every repo has. Until now every docker build sent the whole working tree as its build context, and the image's source archive is made from that context. It now keeps out .git, local build and test output, archives, local databases, .env and a local Go workspace. Every tracked file stays in, so the format check, the linter, the tests, the build and the source archive read the same files as before.

Without .git in the context the binary's build info records no git revision; nothing in routewatch reads it.

Model: opus-5-5

TODO.md

@@ -24,12 +24,13 @@ https://git.eeqj.de/sneak/routewatch/pulls/6. After that, setting
 routewatch up under upaas on fsn1app1 and deploying it are his
 (https://git.eeqj.de/sneak/routewatch/issues/31), and so is the run under a
 real 5 GiB limit (https://git.eeqj.de/sneak/routewatch/issues/3).
-Nothing on the record orders
-https://git.eeqj.de/sneak/routewatch/issues/39 and
-https://git.eeqj.de/sneak/routewatch/issues/30.
+The other open issue is https://git.eeqj.de/sneak/routewatch/issues/30.
 
 # Completed Steps
 
+- 2026-09-29: `.dockerignore` keeps `.git`, local build output, local
+  databases and `.env` out of the Docker build context, and so out of the
+  source archive in the image (closes #39)
 - 2026-09-29: README first line names the MIT license and the author;
   the License section now says MIT and links `LICENSE`, and an Author
   section was added; this file brought up to date (closes #38)
@@ -102,4 +103,3 @@ https://git.eeqj.de/sneak/routewatch/issues/30.
 - `/api/v1/stats` answered HTTP 500 after 35 hours on the live feed, seen
   on `3898daa`, which predates the 2026-09-22 in-memory statistics
   (https://git.eeqj.de/sneak/routewatch/issues/30)
-- Add a `.dockerignore` (https://git.eeqj.de/sneak/routewatch/issues/39)