Docker image layer caching in builder and implications when shared between deploys

Questions / Help
1

Hey,

We are using the free builder at the moment from Github CI actions.
Both for prod and pr review app deployments.

I am working on setting up a multi stage docker image to have separate steps for the hex and npm packages.

I am wondering how does the following scenario play out:

  • Change deps on a pr
  • destroy builder on fly to clear the cache
  • Deploy the pr
  • deploy prod

Will in this case the prod deploy use the layer (for anything) that was cached by the pr deployment?
Would the pr docker image/layers impact the prod deployment?

Thank you

2

The caching from the PR deployment can affect the production deployment yep. There’s very little hidden magic to the builder, it’s just a VM with Docker running in it (it’s not refreshed between builds or anything).

This begs the question of what’s going on where deploying a PR might cache something you don’t want cached - I’m guessing that’s related to the multi-stage part?

What I’ve done is set a build argument in a stage, and then changed/used a specific build argument if I needed to bust the cache of that build step - perhaps depending on staging vs production, etc.

I think something like this might work:

###
# Build the foobar command
##
FROM golang:1.19 as some-client

# >>>>>>>>>>>>>>>>>>> Here's the build argument I'm talking about
ARG BUILD_ENV="staging"

WORKDIR /opt

# >>>>>>>>>>>>>>>>>>> Cache busted here
RUN echo "building $BUILD_ENV"

RUN --mount=type=secret,id=secrets,dst=/tmp/secrets bash -c 'source /tmp/secrets \
    && git clone --single-branch --branch main https://$GITHUB_ACCESS_TOKEN:x-oauth-basic@github.com/foo/bar.git /opt'
RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -a -installsuffix cgo -o bin/foobar cmd/main.go

###
# Put everything together in last stage
##
FROM ubuntu:22.04
COPY --from=some-client /opt/bin/foobar /usr/local/bin/foobar

# and so on...
3

Hey,

Thanks for the reply!

I looked into docker caching, this is the simplest and most relevant way of explanation: caching - How does Docker know when to use the cache during a build and when not? - Stack Overflow

If I have my package.json and my mix.exs files as the first copy statement of their respective steps, I think docker should just figure it out.

So if my build step is a child to the dependency steps, or use things from there docker should know when to use the cache and when not to.

I will try to verify this behavior and update this post accordingly.

EDIT:
This seems to work nicely!

4

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