Add fakes library: recursive JSON template engine, i18n locales, Docker workflow

Dockerfile

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+# Reproducible test/build image. `docker build .` fails if vet or tests fail,
+# so it doubles as CI. Day-to-day, prefer the Makefile (bind-mounts source,
+# no rebuilds). Pin matches the latest stable Go at time of writing.
+FROM golang:1.26.4
+
+WORKDIR /app
+
+# Module layer cached separately from source.
+COPY go.mod ./
+RUN go mod download
+
+COPY . .
+RUN go vet ./... && go test ./...