Trying Out container-structure-test on a Docker Image

I discovered container-structure-test, a tool for verifying whether the structure and settings within a Docker image are as expected.

container-structure-test GitHub Repository

Decided to give it a try.

Setup

First, install container-structure-test. For MacOS, it can be easily installed using Homebrew.

brew install container-structure-test

Next, prepare a Dockerfile for building the test target Docker image as shown below and build the image.

# ./Dockerfile
# @see: https://github.com/silverbirder/testcontainers-nextjs/blob/main/Dockerfile

# ...

##### RUNNER

FROM --platform=linux/amd64 node:18.17-alpine3.18 AS runner
WORKDIR /app

ENV NODE_ENV production

# ENV NEXT_TELEMETRY_DISABLED 1

RUN addgroup --system --gid 1001 nodejs
RUN adduser --system --uid 1001 nextjs

COPY --from=builder /app/next.config.js ./
COPY --from=builder /app/public ./public
COPY --from=builder /app/package.json ./package.json

COPY --from=builder --chown=nextjs:nodejs /app/.next/standalone ./
COPY --from=builder --chown=nextjs:nodejs /app/.next/static ./.next/static

USER nextjs
EXPOSE 3000
ENV PORT 3000

CMD ["node", "server.js"]

The build command is as follows:

docker build . -t app:latest

This command builds the Docker image with the tag app:latest.

Testing with container-structure-test

container-structure-test allows for executing various tests on the built Docker image. You can start the tests with the following command:

container-structure-test test \
  --image app:latest \
  --platform linux/amd64 \
  --config config.yml

config.yml contains the settings for the tests you want to execute. Below is an example:

# config.yml
schemaVersion: "2.0.0"
commandTests:
  - name: "cat server.js"
    command: "cat"
    args: ["server.js"]
    expectedOutput: [".*startServer.*"]
fileContentTests:
  - name: "package.json"
    expectedContents: ["testcontainers-nextjs"]
    path: "/app/package.json"
fileExistenceTests:
  - name: "package.json"
    path: "/app/package.json"
    shouldExist: true
    permissions: '-rw-r--r--'
    uid: 1001
    gid: 1001
metadataTest:
  envVars:
    - key: PORT
      value: 3000
    - key: NODE_ENV
      value: production
  exposedPorts: ["3000"]
  cmd: ["node", "server.js"]
  workdir: "/app"
  user: "nextjs"
# licenseTests:
# - debian: true
#   files: ["/foo/bar", "/baz/bat"]

container-structure-test can execute the following types of tests:

• commandTests
  • Executes commands from the image and verifies if the expected output is obtained.
• fileContentTests
  • Tests whether specific files contain the expected content.
• fileExistenceTests
  • Verifies the existence, permissions, and ownership of files.
• metadataTest
  • Validates metadata such as environment variables, exposed ports, the executing user, etc.
• licenseTests
  • Checks the list of copyright files to ensure that only licenses approved by Google are used.

For more details, please visit the container-structure-test GitHub Repository.

commandTests seem to create a Docker container to execute the CMD for testing. It looks promising for ensuring that errors, warnings, or security logs do not occur.

fileContentTests didn't immediately suggest a use case.

fileExistenceTests seem useful for checking things that should not be allowed by verifying permissions, uid, and gid.

metadataTest appears suitable for testing the application's necessary environment variables with regular expressions.

licenseTests did not immediately suggest a use case either.

Conclusion

container-structure-test is a tool for automatically verifying whether a Docker image meets the expected specifications. Take advantage of the basic usage and test examples introduced in this article to build a safe and reliable container environment.

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