Run Dockerfile build, test and lint steps at nice -n 19 (closes #63)

The owner ordered every build, test and lint step in every Dockerfile to
run at the lowest priority. REPO_POLICIES.md gains that rule in one
line, and both Dockerfiles this repo hands out follow it: its own, and
the canonical Go example in REPO_POLICIES.md. Installs, dependency
downloads and the runtime entrypoint are left at normal priority.

Model: opus-5-5

TODO.md

@@ -21,6 +21,10 @@ fmt-check, and commit.
 
 # Completed Steps
 
+- 2026-09-29: Every `Dockerfile` step that builds, tests or lints now runs at
+  `nice -n 19`, per the owner's order on issue 63: the rule is one line in
+  `REPO_POLICIES.md`, and this repo's `Dockerfile` and the canonical Go
+  `Dockerfile` there follow it.
 - 2026-09-08: Moved linting and testing into Docker as phases of the main
   `Dockerfile`, per the owner ruling on issue 40. `script/lint` and
   `script/test` build one phase each by name with `--no-cache` — the same answer