build(dockerfile): use multi-stage build; reduce size by ~28%

Dockerfile

@@ -1,20 +1,16 @@
-FROM node:20-bullseye-slim
+# ------------------
+# Temp image
+# ------------------
+FROM node:20-bullseye-slim AS tmp
 
 # Workdir
-WORKDIR /usr/app
-
-# Create temp folder for files to be stored whilst being converted
-RUN mkdir -p ./dist/temp/
+WORKDIR /usr/app/tmp
 
-# Install OS dependencies
-# Curl needed for healthcheck command
-RUN apt-get -q update &&\
-    apt-get -y --no-install-recommends install curl poppler-data poppler-utils unrtf && \
-    apt-get clean && \
-    rm -rf /var/lib/apt/lists/*
-
-# Copy and install node dependencies
-COPY package.json package-lock.json ./
+# Copy and install dependencies separately from the app's code
+# to take advantage of Docker's layer caching
+# See: https://docs.docker.com/build/cache/#optimizing-how-you-use-the-build-cache
+COPY package.json .
+COPY package-lock.json .
 RUN npm ci --ignore-scripts --omit=dev && \
     npm pkg delete commitlint devDependencies jest nodemonConfig scripts && \
     npm cache clean --force && \
@@ -27,8 +23,27 @@ RUN npm ci --ignore-scripts --omit=dev && \
 
 # Copy source
 COPY . .
-# Change ownership of all files and directories
-RUN chown -R node:node .
+
+# Create temp folder for files to be stored whilst being converted
+RUN mkdir -p ./dist/temp/
+
+# ------------------
+# Final image
+# ------------------
+FROM node:20-bullseye-slim AS main
+
+# Workdir
+WORKDIR /usr/app
+
+# Install OS dependencies
+# Curl needed for healthcheck command
+RUN apt-get -q update &&\
+    apt-get -y --no-install-recommends install curl poppler-data poppler-utils unrtf && \
+    apt-get clean && \
+    rm -rf /var/lib/apt/lists/*
+
+# Copy files from tmp image and change ownership
+COPY --from=tmp --chown=node:node /usr/app/tmp /usr/app
 
 # Node images provide 'node' unprivileged user to run apps and prevent
 # privilege escalation attacks