Speed up Docker rebuilds

Dockerfile

@@ -1,9 +1,11 @@
+# syntax=docker/dockerfile:1.7
+
 # --- Builder Stage ---
 FROM ubuntu:22.04 AS builder
 ENV DEBIAN_FRONTEND=noninteractive
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
-    build-essential cmake ninja-build git pkg-config \
+    build-essential ccache cmake ninja-build git pkg-config \
     libssl-dev zlib1g-dev \
     libmariadb-dev libmariadb-dev-compat \
     libsqlite3-dev python3 libjsoncpp-dev uuid-dev ca-certificates \
@@ -14,7 +16,10 @@ RUN rm -rf /usr/include/mysql && ln -s /usr/include/mariadb /usr/include/mysql
 WORKDIR /app
 COPY . /app
 
-RUN cmake -S . -B build -G Ninja -DCMAKE_BUILD_TYPE=Release \
+RUN --mount=type=cache,target=/app/.cache/cpm \
+    --mount=type=cache,target=/root/.cache/ccache \
+    cmake -S . -B build -G Ninja -DCMAKE_BUILD_TYPE=Release \
+        -DCMAKE_CXX_COMPILER_LAUNCHER=ccache \
     && cmake --build build
 
 # --- Runtime Stage ---

README.md

@@ -147,6 +147,9 @@ Set `DB_NAME`, `DB_USER`, `DB_PASSWORD`, `MYSQL_ROOT_PASSWORD` and
 origin without quotes or a trailing slash, for example
 `https://visione.studentindipendenti.it`.
 
+Docker builds use BuildKit cache mounts for CPM dependencies and `ccache`, so
+repeat Dokploy builds can reuse Drogon and C++ compilation cache.
+
 ## Current Endpoints
 
 - `GET /api/v1/health`: process health check, no external dependency.