Docker Guide for Beginners/

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Taking a Closer Look at the Docker Image

Taking a Closer Look at the Docker Image

Examine the Dockerfile of the containerized application.

We'll cover the following...

Now that the application is containerized, take a closer look at how some of the machinery works.

Examining the build process

The docker build command parses the Dockerfile one line at a time, starting from the top. We can insert comments by starting a line with the # character, and the builder will ignore them.

Shell-20
ARG NODE_VERSION=20.8.0
FROM node:${NODE_VERSION}-alpine
# Use production node environment by default
ENV NODE_ENV production
WORKDIR /usr/src/app
# Download dependencies as a separate step to take advantage of Docker's caching
# Leverage a cache mount to /root/.npm to speed up subsequent builds
# Leverage a bind mounts to package.json and package-lock.json to avoid having to copy them into
# into this layer
RUN --mount=type=bind,source=package.json,target=package.json \
    --mount=type=bind,source=package-lock.json,target=package-lock.json \
    --mount=type=cache,target=/root/.npm \
    npm ci --omit=dev
# Run the application as a non-root user.
USER node
# Copy the rest of the source files into the image.
COPY . .
# Expose the port that the application listens on.
EXPOSE 8080
# Run the application.
CMD node app.js

All non-comment lines are called instructions or steps and take the format <INSTRUCTION> <arguments>. Instruction names are not case-sensitive, but it’s common to write them in UPPERCASE to make them easier to ...