FROM ubuntu:20.04

# ARG: Build-time variable, not available in the running container by default
ARG APP_VERSION=1.0.0
ARG BUILD_DATE="$(date -u +"%Y-%m-%dT%H:%M:%SZ")"

# ENV: Runtime environment variable, also available during build after its declaration
ENV PORT=8080 \
    DEBUG_MODE=false

LABEL version="$APP_VERSION" \
      build_date="$BUILD_DATE"

# Example of using ARG during build
RUN echo "Building application version $APP_VERSION on $BUILD_DATE"

# Example of using ENV after declaration
RUN echo "Application will run on port $PORT"

# You can pass build args with --build-arg during `docker build`
# Example: docker build --build-arg APP_VERSION=1.1.0 -t myapp .

# You can override ENV vars at runtime with -e during `docker run`
# Example: docker run -e PORT=3000 myapp