Test

This section focuses on testing. The example in this section focuses on linting, but the same principles apply for other kinds of tests as well, such as unit tests. Code linting is a static analysis of code that helps you detect errors, style violations, and anti-patterns.

The exact steps for how to test your code can vary a lot depending on the programming language or framework that you use. The example application used in this guide is written in Go. You will add a build step that uses golangci-lint, a popular linters runner for Go.

Run tests

The golangci-lint tool is available as an image on Docker Hub. Before you add the lint step to the Dockerfile, you can try it out using a docker run command.

$ docker run -v $PWD:/test -w /test \
  golangci/golangci-lint golangci-lint run

You will notice that golangci-lint works: it finds an issue in the code where there's a missing error check.

cmd/server/main.go:23:10: Error return value of `w.Write` is not checked (errcheck)
		w.Write([]byte(translated))
		      ^

Now you can add this as a step to the Dockerfile.

  # syntax=docker/dockerfile:1
  ARG GO_VERSION=1.21
+ ARG GOLANGCI_LINT_VERSION=v1.52
  FROM golang:${GO_VERSION}-alpine AS base
  WORKDIR /src
  RUN --mount=type=cache,target=/go/pkg/mod/ \
      --mount=type=bind,source=go.sum,target=go.sum \
      --mount=type=bind,source=go.mod,target=go.mod \
      go mod download -x

  FROM base AS build-client
  RUN --mount=type=cache,target=/go/pkg/mod/ \
      --mount=type=bind,target=. \
      go build -o /bin/client ./cmd/client

  FROM base AS build-server
  ARG APP_VERSION="0.0.0+unknown"
  RUN --mount=type=cache,target=/go/pkg/mod/ \
      --mount=type=bind,target=. \
      go build -ldflags "-X main.version=$APP_VERSION" -o /bin/server ./cmd/server

  FROM scratch AS client
  COPY --from=build-client /bin/client /bin/
  ENTRYPOINT [ "/bin/client" ]

  FROM scratch AS server
  COPY --from=build-server /bin/server /bin/
  ENTRYPOINT [ "/bin/server" ]

  FROM scratch AS binaries
  COPY --from=build-client /bin/client /
  COPY --from=build-server /bin/server /
+
+ FROM golangci/golangci-lint:${GOLANGCI_LINT_VERSION} as lint
+ WORKDIR /test
+ RUN --mount=type=bind,target=. \
+     golangci-lint run

The added lint stage uses the golangci/golangci-lint image from Docker Hub to invoke the golangci-lint run command with a bind-mount for the build context.

The lint stage is independent of any of the other stages in the Dockerfile. Therefore, running a regular build won’t cause the lint step to run. To lint the code, you must specify the lint stage:

$ docker build --target=lint .

Export test results

In addition to running tests, it's sometimes useful to be able to export the results of a test to a test report.

Exporting test results is no different to exporting binaries, as shown in the previous section of this guide:

  1. Save the test results to a file.
  2. Create a new stage in your Dockerfile using the scratch base image.
  3. Export that stage using the local exporter.

The exact steps on how to do this is left as a reader's exercise :-)

Summary

This section has shown an example on how you can use Docker builds to run tests (or as shown in this section, linters).

Next steps

The next topic in this guide is multi-platform builds, using emulation and cross-compilation.

Multi-platform