Docker local vs. remote cache (--cache-from)

Question

I'm trying to optimize storage in my docker registry and getting problem using remote cache.

My docker file looks as follows:

FROM java:11-debian

LABEL maintainer="Me"
ENV LANG en_US.utf8

RUN addgroup --system --gid 51170 myusername

...

If I build locally multiple times cache is being used.

For Jenkins pipeline I always tag my last build with latest and pull the previous latest just before build of the current message:

docker pull artifactory/image:latest
docker build -t artifactory/image --cache-from artifactory/image:latest .
docker tag artifactory/image artifactory/image:latest
docker push artifactory/image:latest

Log output looks as follows:

 ...
   ---> f6363f58bc32
 Step 2/18 : LABEL maintainer="Me"
   ---> Using cache
   ---> 54efd5e63c04
 Step 3/18 : ENV LANG en_US.utf8
   ---> Using cache
   ---> 6865c5539193
 Step 4/18 : RUN addgroup --system --gid 51170 myusername
  Removing intermediate container f8d05d2e5f16
   ---> af5202782abc
...

Why RUN is not cached in case of remote cache? What are the differences between local cache and usage of --cache-from?

Answer 1

From https://docs.docker.com/engine/reference/commandline/build/#specifying-external-cache-sources

To use an image as a cache source, cache metadata needs to be written into the image on creation. This can be done by setting --build-arg BUILDKIT_INLINE_CACHE=1 when building the image. After that, the built image can be used as a cache source for subsequent builds.

Additionally, to use the Docker BuildKit, you must have DOCKER_BUILDKIT=1 in your environment. The easiest method being

DOCKER_BUILDKIT=1 docker build ...