Implement Docker rootless user with reviewer feedback improvements

This commit implements the rootless user functionality for both production
and development Docker images with all improvements from PR #837 review:

Changes made:
- Added rootless user setup using 'rails' username (not 'rails_api_base')
- Used ARG instead of ENV for USERNAME, USER_UID, and USER_GID build variables
- Set proper file ownership and permissions (700) for security
- Added explanatory comment for temporary root switch in jemalloc symlink creation
- Applied --chown and --chmod flags to all COPY operations

Security improvements:
- Application now runs as non-root user (UID/GID 1000)
- Reduced attack surface by limiting privileges
- Follows Docker security best practices

Files modified:
- Dockerfile: Added rootless user for production image
- Dockerfile.dev: Added rootless user for development image

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: claude

Dockerfile

@@ -61,19 +61,36 @@ RUN apt-get update -qq && \
     apt-get install --no-install-recommends -y curl libpq-dev libvips libjemalloc2 libyaml-dev && \
     apt-get clean && rm -rf /var/lib/apt/lists/*
 
-# Create app directory.
-RUN mkdir -p "${APP_HOME}"
+# Setup rootless user for security
+ARG USERNAME=rails
+ARG USER_UID=1000
+ARG USER_GID=1000
+
+RUN groupadd --gid $USER_GID $USERNAME && \
+    useradd --uid $USER_UID --gid $USER_GID -m $USERNAME
+
+# Create app directory with proper ownership
+RUN mkdir -p "${APP_HOME}" && \
+    chown -R $USERNAME:$USERNAME $APP_HOME && \
+    chmod -R 700 $APP_HOME
+
+# Switch to non-root user
+USER $USERNAME
 
 # Setup work directory.
 WORKDIR $APP_HOME
 
-# Copy everything from the builder image
-COPY --link . .
-COPY --from=builder $APP_HOME/public/ $APP_HOME/public/
-COPY --from=builder $APP_HOME/tmp/ $APP_HOME/tmp/
-COPY --from=builder $APP_HOME/vendor/ $APP_HOME/vendor/
+# Copy everything from the builder image with proper ownership
+COPY --link --chown=$USERNAME:$USERNAME --chmod=700 . .
+COPY --from=builder --chown=$USERNAME:$USERNAME --chmod=700 $APP_HOME/public/ $APP_HOME/public/
+COPY --from=builder --chown=$USERNAME:$USERNAME --chmod=700 $APP_HOME/tmp/ $APP_HOME/tmp/
+COPY --from=builder --chown=$USERNAME:$USERNAME --chmod=700 $APP_HOME/vendor/ $APP_HOME/vendor/
 
+# Temporarily switch to root to create system-level symlink for jemalloc
+# This is required because symlink creation in /usr/lib requires root privileges
+USER root
 RUN ln -s /usr/lib/*-linux-gnu/libjemalloc.so.2 /usr/lib/libjemalloc.so.2
+USER $USERNAME
 
 # Deployment options
 ENV RAILS_LOG_TO_STDOUT=true

Dockerfile.dev

@@ -20,24 +20,37 @@ ENV WORK_ROOT=/src
 ENV APP_HOME=$WORK_ROOT/app/
 ENV LANG=C.UTF-8
 
-# Create app directory.
-RUN mkdir -p $APP_HOME
+# Setup rootless user for security
+ARG USERNAME=rails
+ARG USER_UID=1000
+ARG USER_GID=1000
+
+RUN groupadd --gid $USER_GID $USERNAME && \
+    useradd --uid $USER_UID --gid $USER_GID -m $USERNAME
+
+# Create app directory with proper ownership
+RUN mkdir -p $APP_HOME && \
+    chown -R $USERNAME:$USERNAME $APP_HOME && \
+    chmod -R 700 $APP_HOME
+
+# Switch to non-root user
+USER $USERNAME
 
 # Setup work directory.
 WORKDIR $APP_HOME
 
 RUN gem install foreman bundler
 
 # Copy dependencies files and install libraries.
-COPY --link package.json yarn.lock .yarnrc.yml ./
+COPY --link --chown=$USERNAME:$USERNAME --chmod=700 package.json yarn.lock .yarnrc.yml ./
 
 RUN corepack enable
 RUN yarn install --immutable && yarn cache clean
 
-COPY --link Gemfile Gemfile.lock .ruby-version ./
+COPY --link --chown=$USERNAME:$USERNAME --chmod=700 Gemfile Gemfile.lock .ruby-version ./
 RUN bundle install -j 4
 
-COPY --link . .
+COPY --link --chown=$USERNAME:$USERNAME --chmod=700 . .
 
 RUN yarn build