examples: add Docker build example (Kagami/go-face#76)

Multi-stage build: compiles dlib from source on Alpine (which has no
dlib package), builds the detection example against it (dlib links
statically, so the runtime stage doesn't need it), and produces a
~50MB runtime image with just the shared libraries the compiled binary
actually needs (found via a missing-library failure on the first
attempt: cblas is a separate runtime package from blas on Alpine, only
pulled in transitively by blas-dev at build time).

Includes two patches recurring across the upstream install-error
reports (e.g. Kagami/go-face#75, #79, #83, #87): dlib's released source
doesn't build with GCC 15+ (missing an implicit <cstdint> include) or
CMake 4.x (rejects dlib's old cmake_minimum_required() declarations)
without them.

Verified by actually building and running the image: all 7 faces in
elenco3.jpg detected and labeled correctly inside the container.

Author: Leandro

README.md

@@ -93,6 +93,14 @@ Try installing dlib/libjpeg with your distribution's package manager, or
 with old dlib packages such as libdlib18. If your system isn't covered here,
 open an issue with the distribution/version and we'll try to help.
 
+### Docker
+
+[`examples/Dockerfile`](examples/Dockerfile) builds dlib from source (Alpine
+has no dlib package) and compiles the detection example against it, ending
+with a ~50MB runtime image. Useful as a reference for containerized builds,
+and for the compiler/CMake compatibility patches it applies -- dlib's
+released source doesn't build out of the box with GCC 15+ or CMake 4.x.
+
 ## Installation
 
 ```bash

examples/Dockerfile

@@ -0,0 +1,70 @@
+# Example multi-stage build: compiles dlib from source (Alpine has no dlib
+# package), builds the detection example against it, and produces a slim
+# runtime image with only the shared libraries the compiled binary needs.
+#
+# Build:
+#   docker build -t go-recognizer-example -f examples/Dockerfile .
+# Run (mount your own fotos/models directories):
+#   docker run --rm \
+#     -v "$(pwd)/examples/fotos":/data/fotos:ro \
+#     -v "$(pwd)/examples/models":/data/models:ro \
+#     -v "$(pwd)/out":/data \
+#     go-recognizer-example
+
+# ---- dlib: compile dlib from source ----
+FROM golang:1.25-alpine AS dlib
+
+RUN apk add --no-cache \
+    cmake make g++ libstdc++ libgcc \
+    jpeg-dev libpng-dev giflib-dev libjpeg-turbo-dev \
+    blas-dev lapack-dev \
+    ca-certificates wget
+
+WORKDIR /
+ARG DLIB_VERSION=v19.24.8
+
+RUN wget -q https://github.com/davisking/dlib/archive/${DLIB_VERSION}.tar.gz \
+ && tar xf ${DLIB_VERSION}.tar.gz \
+ && mv dlib-* dlib \
+ && mkdir -p dlib/build \
+ # GCC 15+ dropped some implicit <cstdint> includes dlib relies on.
+ && sed -i '1i#include <cstdint>' dlib/dlib/serialize.h \
+ # CMake 4.x refuses dlib's old cmake_minimum_required() declarations.
+ && find dlib -type f \( -name "CMakeLists.txt" -o -name "*.cmake" \) \
+      -exec sed -i -E 's/cmake_minimum_required\s*\(\s*VERSION\s+[0-9.]+\s*\)/cmake_minimum_required(VERSION 3.10)/gI' {} + \
+ && cd dlib/build \
+ && cmake -DCMAKE_POLICY_VERSION_MINIMUM=3.10 \
+          -DCMAKE_BUILD_TYPE=Release \
+          -DDLIB_PNG_SUPPORT=ON \
+          -DDLIB_GIF_SUPPORT=ON \
+          -DDLIB_JPEG_SUPPORT=ON .. \
+ && cmake --build . --config Release -- -j"$(nproc)" \
+ && make install \
+ && cd / && rm -rf dlib ${DLIB_VERSION}.tar.gz
+
+# ---- builder: compile the Go example against dlib ----
+FROM dlib AS builder
+
+WORKDIR /src
+COPY examples/ ./examples/
+
+WORKDIR /src/examples
+RUN go mod tidy \
+ && CGO_ENABLED=1 go build -o /out/detection ./detection
+
+# ---- runtime: slim image with just the compiled binary ----
+FROM alpine:latest
+
+# blas/cblas/lapack, libjpeg-turbo/libpng/giflib: dlib's runtime
+# dependencies. dlib itself is statically linked into the binary (the
+# cmake build above produces libdlib.a, not a .so), so it isn't needed
+# here.
+RUN apk add --no-cache \
+    libstdc++ libgomp \
+    libjpeg-turbo libpng giflib \
+    blas cblas lapack
+
+COPY --from=builder /out/detection /usr/local/bin/detection
+
+WORKDIR /data
+ENTRYPOINT ["/usr/local/bin/detection"]