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

@@ -1,46 +1,9 @@
-FROM rocker/shiny:4.5.2
+FROM standrews-base:latest
 
-# 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 first for layer caching
-COPY renv.lock .
-
-# 3. Install packages into site-library (accessible to all users including shiny)
-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)"
-
-# 4. Copy application code
+# Copy application code
 COPY . .
 
-# 5. Permissions
+# Permissions
 RUN chown -R shiny:shiny /srv/shiny-server
 
 EXPOSE 3838