Format the Markdown with prettier in make fmt and make fmt-check (closes #215)

make fmt and make fmt-check covered only Go, so Markdown formatting was
checked by eye. prettier, pinned in package.json and yarn.lock beside
ESLint and installed by the same js-deps stage, now formats every
Markdown file with the settings in .prettierrc (4-space tabs, prose
wrapped). It runs only in Docker: make fmt writes the formatted files
back from the markdown-output stage, and make fmt-check and the image
build run the markdown-check stage. The Dockerfile's lint stage now
runs the gofmt check itself, since make fmt-check needs a docker daemon.

Model: opus-5-5

.dockerignore

@@ -15,8 +15,8 @@ bin/
 # Extracted from 3p/ by `make assets` inside the build; a host copy is not
 # needed. The tarball in 3p/ must stay in the context.
 static/js/alpine.min.js
-# The js-deps stage installs ESLint; a host copy would overwrite it at the
-# js-lint stage's `COPY . .`.
+# The js-deps stage installs ESLint and prettier; a host copy would overwrite
+# them at the `COPY . .` of the stages built on it.
 node_modules/
 .env
 .env.*