doc: update docker file to incldue multi-stage build from source with enterprise bypass

tools/fleet-docker/Dockerfile

@@ -1,3 +1,23 @@
+# Build stage — compiles Fleet from source
+FROM golang:1.26.5-alpine AS builder
+ARG FLEET_VERSION=unknown
+
+WORKDIR /build
+
+RUN apk add --no-cache git ca-certificates gcc musl-dev make
+
+COPY go.mod go.sum ./
+RUN go mod download
+
+COPY . .
+
+RUN CGO_ENABLED=1 GOOS=linux go build -o fleet \
+    -tags=full,fts5,netgo \
+    -trimpath \
+    -ldflags="-s -w -extldflags '-static'" \
+    ./cmd/fleet
+
+# Runtime stage
 FROM alpine:3.23.4@sha256:5b10f432ef3da1b8d4c7eb6c487f2f5a8f096bc91145e68878dd4a5019afde11
 ARG FLEET_VERSION=unknown
 LABEL maintainer="Fleet Developers"
@@ -15,8 +35,11 @@ RUN apk --no-cache upgrade openssl libcrypto3 libssl3
 # Create fleet group and user
 RUN addgroup -S fleet && adduser -S fleet -G fleet
 
+# Create installers directory
+RUN mkdir -p /opt/fleet/installers && chown -R fleet:fleet /opt/fleet
+
 USER fleet
 
-COPY fleet /usr/bin/
+COPY --from=builder /build/fleet /usr/bin/fleet
 
 CMD ["fleet", "serve"]