docs: remove goofy sections from README

Dockerfile

@@ -1,11 +1,30 @@
-FROM golang:1.23-alpine
+# syntax=docker/dockerfile:1
 
-WORKDIR /app
+FROM --platform=$BUILDPLATFORM golang:1.23-alpine AS build
+
+WORKDIR /src
+ENV GOTOOLCHAIN=local
+
+RUN apk add --no-cache ca-certificates
+
+COPY go.mod go.sum ./
+RUN --mount=type=cache,target=/go/pkg/mod go mod download
 
 COPY . .
 
-RUN go mod init main
-RUN go mod tidy
-RUN go build -o app .
+ARG TARGETOS=linux
+ARG TARGETARCH
+RUN --mount=type=cache,target=/go/pkg/mod \
+    --mount=type=cache,target=/root/.cache/go-build \
+    CGO_ENABLED=0 GOOS=$TARGETOS GOARCH=${TARGETARCH:-$(go env GOARCH)} \
+    go build -trimpath -tags netgo -ldflags="-s -w" -o /out/overleaf-registration-be .
 
-CMD ["./app"]
+FROM scratch AS runtime
+
+COPY --from=build /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/
+COPY --from=build /out/overleaf-registration-be /overleaf-registration-be
+
+USER 65532:65532
+EXPOSE 3000
+
+ENTRYPOINT ["/overleaf-registration-be"]