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DazWilkin and ImJasonH [Go] Modules sharing packages example (#518)
* Attempt to clarify Modules and Mirror

* Reflect @ImJasonH feedback

* Added "multi_step" example

* Undo stoopid
Latest commit cf071bc Jul 10, 2019

Tool builder:

This builder runs the go tool (go build, go test, etc.) after placing source in /workspace into the GOPATH before running the tool.

Using golang and Go Modules

This Builder ( is not necessary if you're building using Go modules, available in Go 1.11+. You can build with the golang image (not from Dockerhub and Google's Container Registry mirror:

# If you already have a go.mod file, you can skip this step.
- name:
  args: ['go', 'mod', 'init', '']

# Build the module.
- name:
  env: ['GO111MODULE=on']
  args: ['go', 'build', './...']

See examples/module for a working example.

Note #1 /workspace and /go

The Golang image defaults to a working directory of /go whereas Cloud Build mounts your sources under /workspace. This reflects the recommend best practice when using Modules of placing your sources outside of GOPATH. When you go build ./... you will run this in the working directory of /workspace but the packages will be pulled into /go/pkg.

Because /go is outside of /workspace, the /go directory is not persisted across Cloud Build steps. See next note.

Note #2 Sharing packages across steps

One advantage with Go Modules is that packages are now semantically versioned and immutable; one a package has been pulled once, it should not need to be pulled again. Because the golang image uses /go as its working directory and this is outside of Cloud Build's /workspace directory, /go is recreated in each Golang step. To avoid this and share packages across steps, you may use Cloud Build volumes. An example to prove the point:

- name:
  - GO111MODULE=on
  args: ['go','get','-u','']
  - name: go-modules
    path: /go

- name: golang
  - GO111MODULE=on
  args: ['go','list','-f','{{ .Dir }}','-m','']
  - name: go-modules
    path: /go

In the above, if the volumes section were omitted, the second step would fail. This is because /go would be created anew for the step and would not be present in it.

NB Cloud Build supports using build-wide settings for env and volumes using options (see link). I've duplicated here to aid clarity.

See examples/multi_step for a working example.

Note #3 Golang Module Mirror

The Go team provides a Golang Module Mirror ( You may utilize the Mirror by including GOPROXY= in your build steps, e.g.:

- name:
  - GO111MODULE=on
  args: ['go','get','-u','']
  - name: go-modules
    path: /go


alpine vs debian

There are two supported versions of this builder, one for alpine and one for debian. The difference is significant since, by default, Go dynamically links libc. Binaries built in an alpine environment don't always work in a debian-like (including ubuntu, etc) environment, or vice versa.

The specific versions are available as


And is an alias for

Workspace setup

Before the go tool is used, the build step first sets up a workspace.

To determine the workspace structure, this tool checks the following, in order:

  1. Is $GOPATH set? Use that.
  2. Is $PROJECT_ROOT set? Make a temporary workspace in GOPATH=./gopath, and link the contents of the current directory into ./gopath/src/$PROJECT_ROOT/*.
  3. Is there a ./src directory? Set GOPATH=$PWD.
  4. Does a .go file in the current directory have a comment like // import "$PROJECT_ROOT"? Use the $PROJECT_ROOT found in the import comment.

Once the workspace is set up, the args to the build step are passed through to the go tool.

This tool builder sets CGO_ENABLED=0, so that all binaries are linked statically.

Output files

The binaries built by a go install step will be available to subsequent build steps.

If you use the install subcommand, the binaries will end up in $GOPATH/bin.

  • If you set $GOPATH, the binaries will end up in $GOPATH/bin.
  • If you rely on ./src to indicate the workspace, the binaries will end up in ./bin.
  • If you use $PROJECT_ROOT or an //import comment, the binaries will end up in ./gopath/bin.


  • Hello, World! is a basic example that builds a binary that is injected into a container image. It uses the $PROJECT_ROOT method to define its workspace.
  • Whole workspace is the same as the "Hello, World!" example, except that it uses the ./src method to define its workspace.

Building this builder

To build this builder, run the following command in this directory.

$ gcloud builds submit . --config=cloudbuild.yaml
You can’t perform that action at this time.