Golang Docker Dependency Cache

If you've switched to go modules you might have found your docker builds have slowed down due go build needing to recompile all the dependencies even if they have already been downloaded or vendored.

For the simple program in this repo with a single dependency I was able to get 4x improvement over doing no dependency caching and 3x compared to using go mod download as the dependency cache.

There is an open issue but until it's built into the go cli this could make your builds much faster.

Usage

COPY go.mod go.sum ./

# Add this line before `go build`
RUN go mod graph | awk '{if ($1 !~ "@") print $2}' | xargs go get

Full Dockerfile example: ./Dockerfile

Benchmarks

I used time to measure how long it took to build a fresh docker image with --no-cache and then changed the main.go file by adding a comment so Docker would run the go build step again while keeping the preceeding cached docker layers.

With go mod graph | awk '{if ($1 !~ "@") print $2}' | xargs go get dependency cache:

# Fresh build with no cache:
$ time docker build --no-cache -t golang-docker-cache .
...
13.54 real         0.12 user         0.07 sys

# Add comment to main.go:
$ time docker build -t golang-docker-cache .
2.74 real         0.12 user         0.10 sys

With go mod download dependency cache:

# Fresh build with no cache:
$ time docker build --no-cache -t golang-docker-cache .
15.13 real         0.13 user         0.09 sys

# Add comment to main.go:
$ time docker build -t golang-docker-cache .
9.03 real         0.12 user         0.10 sys

Without any dependency cache:

# Fresh build with no cache:
$ time docker build --no-cache -t golang-docker-cache .
12.12 real         0.12 user         0.08 sys

# Add comment to main.go:
$ time docker build -t golang-docker-cache .
12.19 real         0.12 user         0.10 sys