Add Docker build script and README

Multi-stage Dockerfile runs the tests, builds a static binary, and ships
it on Alpine with git for GitHub clones.

Dockerfile

@@ -0,0 +1,17 @@
+FROM golang:1.26-alpine AS build
+WORKDIR /src
+COPY go.mod ./
+COPY *.go ./
+COPY web ./web
+RUN CGO_ENABLED=0 go test ./... \
+ && CGO_ENABLED=0 go build -trimpath -ldflags="-s -w" -o /codenavigator .
+
+FROM alpine:3.22
+# git is needed at runtime to clone GitHub repositories.
+RUN apk add --no-cache git ca-certificates && adduser -D -H app
+COPY --from=build /codenavigator /usr/local/bin/codenavigator
+USER app
+ENV PORT=4177
+EXPOSE 4177
+HEALTHCHECK CMD wget -qO- http://127.0.0.1:$PORT/api/health || exit 1
+ENTRYPOINT ["codenavigator"]