Each container built in a FoundriesFactory will publish the build cache layers to our private registry, Subsequent builds will pull from this cache, importing it for the current build. This provides efficient incremental container builds for our FoundriesFactory users. Exporting and importing these build cache layers uses the built-in features of Docker Buildx.

While the cache is very helpful, there are a few things to note:

  • The build cache for each container image is branch specific
  • It is architecture specific
  • It can be invalidated for a few reasons, such as source files changing or a base image update.

Cache Invalidation

When trying to understand why the cache has been invalidated, there are not any tools that can assist. That said, in a FoundriesFactory each container build will automatically use the docker build context to create a list of md5sums of all source files used in the build. Each Compose Apps will generate an artifact named <compose-app-name>-md5sum.txt, which then can be used to generate a diff from build to build to assist in understanding what files may have changed, and how that effects the caching. Generally, each line in your Dockerfile creates it’s own image layer, and a corresponding cache layer. If source files change, or any direct modification to the Dockerfile occurs, it will invalidate the cache from that point forward. With the importance of these concepts towards building Compose Apps, it is recommended to read the documentation related to Dockerfile best practices.