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

script/fmt

@@ -1,5 +1,7 @@
 #!/bin/sh
-# script/fmt: format all files (writes).
+# script/fmt: format all files (writes): the Go code with gofmt and
+# goimports, the Markdown with prettier. prettier is never installed
+# locally: it runs in docker, in the Dockerfile's Markdown stages.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
@@ -10,6 +12,7 @@ main() {
     if command -v goimports >/dev/null 2>&1; then
         goimports -w .
     fi
+    docker build --target markdown-output --output type=local,dest=. .
 }
 
 main "$@"