.dockerignore keeps root test binaries and coverage out (closes #155)

A Go test binary or coverage file left at the repository root, which
.gitignore already ignores, went into the build context of every docker
build in script/ and was copied into the lint, test and build stages.
The repository's own section of .dockerignore now lists /*.test and
/*.out, root-anchored like its other entries.

Checked with a probe image that copies the whole context: x.test and
x.out planted at the root are no longer in it, and nothing else changed.

Model: opus-5-5

TODO.md

@@ -22,6 +22,14 @@ Run `make frontend-viewport-test` in CI as its own step; it is not part of
 
 # Completed Steps
 
+- 2026-10-19: `.dockerignore` keeps Go test binaries and coverage output at the
+  root out of the build context, with `/*.test` and `/*.out` in the repository's
+  own section ([#155](https://git.eeqj.de/sneak/netwatch/issues/155)). Such
+  files went into the build context of every `docker build` in `script/` and
+  were copied into the lint, test and build stages. Checked with a probe image
+  that copies the whole context: `x.test` and `x.out` planted at the root no
+  longer land in it, and nothing else changed. The entries match only at the
+  root, so the same files in a subdirectory still reach the image
 - 2026-10-19: `.gitea/workflows/check.yml` is the sneak/prompts copy at
   `0653579` ([#153](https://git.eeqj.de/sneak/netwatch/issues/153)). Its
   checkout step sets `fetch-depth: 0`, so CI clones all history and tags, and