Collapse fake multi-stage Docker build into single stage

ruby/Dockerfile

@@ -1,17 +1,22 @@
 ARG RUBY_VERSION
-FROM registry.docker.com/library/ruby:$RUBY_VERSION-slim as base
+FROM registry.docker.com/library/ruby:$RUBY_VERSION-slim
 
 # The app lives here
 WORKDIR /app
 
-# Install base packages
+# Install base packages and packages needed to build gems
 RUN apt-get update -qq \
     && apt-get install --no-install-recommends -y \
+    build-essential \
     curl \
+    git \
+    libpq-dev \
+    libyaml-dev \
+    pkg-config \
     postgresql-client \
     && rm -rf /var/lib/apt/lists /var/cache/apt/archives
 
-# Non-root user for runtime stages (USER not set here so build stage keeps root)
+# Non-root user for downstream runtime stages
 RUN useradd --system --uid 1000 app && chown app:app /app
 
 # Set production environment
@@ -20,18 +25,5 @@ ENV BUNDLE_DEPLOYMENT="1" \
     BUNDLE_CACHE_PATH="/usr/local/bundle/cache" \
     BUNDLE_WITHOUT="development"
 
-# Throw-away build stage to reduce size of final image
-FROM base as build
-
-# Install packages needed to build gems
-RUN apt-get update -qq \
-    && apt-get install --no-install-recommends -y \
-    build-essential \
-    git \
-    libpq-dev \
-    libyaml-dev \
-    pkg-config \
-    && rm -rf /var/lib/apt/lists /var/cache/apt/archives
-
 # This can be overwritten at runtime
 CMD ["ruby", "-v"]