Split Dockerfile into base + app layers for fast CI rebuilds

Base image (standrews-base) contains system deps and R packages.
App Dockerfile is now just COPY + permissions — rebuilds in seconds.

Dockerfile.base

@@ -0,0 +1,37 @@
+FROM rocker/shiny:4.5.2
+
+# 1. System dependencies (spatial stack required for sf)
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    build-essential \
+    cmake \
+    curl \
+    libabsl-dev \
+    libcurl4-openssl-dev \
+    libfontconfig1-dev \
+    libfreetype6-dev \
+    libfribidi-dev \
+    libgdal-dev \
+    libgeos-dev \
+    libharfbuzz-dev \
+    libicu-dev \
+    libjpeg-dev \
+    libpng-dev \
+    libproj-dev \
+    libssl-dev \
+    libtiff5-dev \
+    libudunits2-dev \
+    libxml2-dev \
+    pkg-config \
+    proj-bin \
+    proj-data \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /srv/shiny-server
+
+# 2. Copy lockfile and restore R packages
+COPY renv.lock .
+
+ENV RENV_PATHS_LIBRARY=/usr/local/lib/R/site-library
+
+RUN R -q -e "install.packages('renv', repos='https://cloud.r-project.org')" \
+    && R -q -e "renv::restore(library='/usr/local/lib/R/site-library', prompt=FALSE)"