docs: use bind mounts in Go Dockerfile examples

[Juneezee]

Description

I remembered there used to be a page (https://docs.docker.com/build/guide/mounts/) that demonstrated how to use both bind mounts and cache mounts together in a Dockerfile for a Go application. However, that page was removed in #20715, and the new page (https://docs.docker.com/build/cache/optimize/) now separates the examples for bind mounts and cache mounts.

docs/content/build/guide/mounts.md

Lines 185 to 194 in 91b324f

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

It would be great if the remaining Go Dockerfile examples could show how to use both bind mounts and cache mounts together specifically for go.mod and go.sum. This combination was really helpful for improving build performance and clarity when using Docker for Go development. (See #22388 (comment) for updtae)

Related issues or tickets

• build: incorporate build guide into manuals #20715

Reviews

• Technical review
• Editorial review
• Product review

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[Juneezee]

UPDATE: I have removed the go mod download step in commit cf728b4.

go build will automatically download module dependencies. In many cases, that is a much smaller set of modules than what is downloaded by go mod download. I found a great explanation on StackOverflow here: https://stackoverflow.com/a/68172023/7902371

Using the https://github.com/github/github-mcp-server Go application as an example below.

Size of GOMODCACHE with go mod download:

~/Desktop/github/github-mcp-server docker ❯ go clean -i -r -cache -modcache

~/Desktop/github/github-mcp-server docker ❯ go mod download

~/Desktop/github/github-mcp-server docker ❯ du -sh ~/go/pkg/mod
186M	/home/jun/go/pkg/mod

Size of GOMODCACHE with go build:

~/Desktop/github/github-mcp-server docker !1 ❯ go clean -i -r -cache -modcache

~/Desktop/github/github-mcp-server docker !1 ❯ CGO_ENABLED=0 go build -ldflags="-s -w" cmd/github-mcp-server/main.go
go: downloading github.com/spf13/viper v1.20.1
go: downloading github.com/mark3labs/mcp-go v0.18.0
go: downloading github.com/google/go-github/v69 v69.2.0
go: downloading github.com/sirupsen/logrus v1.9.3
go: downloading github.com/spf13/cobra v1.9.1
go: downloading golang.org/x/sys v0.31.0
go: downloading github.com/spf13/afero v1.14.0
go: downloading github.com/fsnotify/fsnotify v1.8.0
go: downloading github.com/spf13/cast v1.7.1
go: downloading github.com/go-viper/mapstructure/v2 v2.2.1
go: downloading github.com/subosito/gotenv v1.6.0
go: downloading gopkg.in/yaml.v3 v3.0.1
go: downloading github.com/spf13/pflag v1.0.6
go: downloading github.com/pelletier/go-toml/v2 v2.2.3
go: downloading github.com/sagikazarmark/locafero v0.9.0
go: downloading golang.org/x/text v0.23.0
go: downloading github.com/google/uuid v1.6.0
go: downloading github.com/yosida95/uritemplate/v3 v3.0.2
go: downloading github.com/sourcegraph/conc v0.3.0
go: downloading github.com/google/go-querystring v1.1.0

~/Desktop/github/github-mcp-server docker !1 ?1 ❯ du -sh ~/go/pkg/mod
80M	/home/jun/go/pkg/mod

Running go build results in a smaller GOMODCACHE (~80 MB) compared to go mod download (~186 MB).

[Juneezee]

Hi @aevesdocker @crazy-max, when you have a moment, could you take a quick look at this PR and let me know if it would be a worthwhile improvement to the documentation?

Thanks 😊

[thaJeztah]

Looks like I wrote some comments last week, but didn't submit them 🙈

[thaJeztah]

Wondering if this one would still be useful; the default would be ~/.cache/go-build, but as the mounts are hard-coded to a location, it could be useful to explicitly configure go to use a location (for the mounts).

Possibly it could be a custom path so that the cache-directory can be set up to a predictable location for situations where you want to run the build as a user (USER) that's not root 🤔

[thaJeztah]

Perhaps @crazy-max or @tonistiigi have ideas on this part; yes, it looks like go mod download may download more; but it could depend on the situation; i.e., if all modules are downloaded, then possibly the cache could be re-used for multiple stages (instead of each stage downloading "just" the parts it needs).

[crazy-max]

For projects that don't vendor the dependencies this command is still useful so we should keep it imo.

the cache could be re-used for multiple stages (instead of each stage downloading "just" the parts it needs).

For multi-stage but also if doing a multiplatform build it will download it once similar to https://github.com/docker/compose/blob/6a2d16bd106bd84065e790b5c2b111509e8a6fd9/Dockerfile#L50-L54

[thaJeztah]

Also double-checking with @crazy-max and @tonistiigi on this one; i.e., if copying the source was intentional here (restoring cache or otherwise?).

These are always tricky as there's no "single" rule, some bits may depend on your situation.

[dvdksn]

I don't recall for sure but I believe that I might've opted for COPY just to simplify the syntax on this page - it's specifically about cache mounts, so I probably wanted to make everything else as simple as possible. But the obvious downside of that is that we don't show the "optimal" pattern of bind mounting, which lead to this PR being raised.

I appreciate that it's easier to have all the best practices shown in the same example so I'm happy to get this in!

[crazy-max]

For projects that don't vendor the dependencies this command is still useful so we should keep it imo.

the cache could be re-used for multiple stages (instead of each stage downloading "just" the parts it needs).

For multi-stage but also if doing a multiplatform build it will download it once similar to https://github.com/docker/compose/blob/6a2d16bd106bd84065e790b5c2b111509e8a6fd9/Dockerfile#L50-L54

[Juneezee]

Hi @thaJeztah and @crazy-max,

It's been a while. I wanted to follow up on this PR since I addressed the previous review comments.

Is there anything else I should do from my side, or do you think the PR is ready for the next review round/merge?

Thank you!

/cc @dvdksn (Author of PR #20715)

[dvdksn]

LGTM thank you @Juneezee