Including pkg in .dockerignore file

Question

Right now my .dockerignore file has this contents:

.vscode
.idea
.git
bin
pkg

and my Dockerfile looks like:

FROM golang:latest
RUN mkdir -p /app
WORKDIR /app
COPY . .
ENV GOPATH /app
RUN go install huru
EXPOSE 3000
ENTRYPOINT /app/bin/huru

My question is - should I be copying the pkg folder from host to image or not? Right now I am not, as my dockerignore file makes clear.

I get the feeling that I should just COPY the pkg folder from host to image, because that might have pre-built files in it that go install can use instead of re-downloading the source from github etc?

Answer 1

Personally, I think copying pkg folder from host to image is not a good idea because :

• it tightly couples the place from where you are building the image (your host) and the image itself. You could potentially have differences in resulting images depending on where you build the image, and that's probably what you don't want
• moreover, if you have automated builds (from CI for example), you're probably rebuilding the whole application from a clean environment each time, so there is no initial pkg folder to copy.

If you're familiar with Java world, I've already encountered that problem for images built with Maven. To speed up the build, some people are copying their local Maven repository (~/.m2) in the image to avoid redownloading artifacts. I don't particularly agree with that, since there is always a risk that their .m2 folder contains corrupted artifacts : therefore, the image built on their machine will be different than if it was built on a clean environment. It depends on whether you want to have consistent builds or quick builds (I prefer the former).

In conclusion, I think that building images from a clean environment, without depending on the host where the image is built, is a good practice. That's why I personally would not copy any files (except application source code!) inside the image.