Does putting ARG at top of Dockerfile prevent layer re-use?

To be more precise than the accepted response, not all lines are cache invalidated after an ARG declaration. Only those that use ARG values and RUNs. The docker documentation the details:

Impact on build caching

ARG variables are not persisted into the built image as ENV variables are. However, ARG variables do impact the build cache in similar ways. If a Dockerfile defines an ARG variable whose value is different from a previous build, then a “cache miss” occurs upon its first usage, not its definition. In particular, all RUN instructions following an ARG instruction use the ARG variable implicitly (as an environment variable), thus can cause a cache miss. All predefined ARG variables are exempt from caching unless there is a matching ARG statement in the Dockerfile.

• https://docs.docker.com/engine/reference/builder/#impact-on-build-caching

You’ll have to move your ARGs under the RUNs that would not need the argument in order to keep layer cache optimized.

For more info:

• https://github.com/moby/moby/issues/18017
• https://github.com/moby/moby/pull/18161
• RUN explanation here: https://github.com/moby/moby/pull/21885