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

Dockerfile

@@ -13,7 +13,7 @@ RUN script/bootstrap
 
 COPY . .
 
-RUN yarn run prettier --check '**/*.md' --tab-width 4 --prose-wrap always
+RUN nice -n 19 yarn run prettier --check '**/*.md' --tab-width 4 --prose-wrap always
 
 # Test phase, same shape and for the same reason.
 #
@@ -28,7 +28,7 @@ RUN script/bootstrap
 
 COPY . .
 
-RUN echo "No tests defined."
+RUN nice -n 19 echo "No tests defined."
 
 # Development environment, and the last stage: a plain `docker build .`
 # names no target and so builds this one. Nothing is wanted from the two