the Docker COPY command should perform a checksum of the given source files, and use that for caching. It should not even check the creation/modification time of the files.
The RUN command should only use the command string to determine whether to use the cache or re-run the command.
Therefore, the dependencies will be re-fetched if any of the following conditions apply:
-
The Dockerfile line
RUN mix deps.getor any Dockerfile command appearing before it was modified -
The content of
mix.exsormix.lockchanged
As I understand, you say that none of those happened, and the dependencies are still re-fetched. In order to understand what is going on, it would be interesting to check:
-
What is the first Dockerfile command to invalidate the cache? Is it
RUN mix deps.getor something even earlier? -
If you separate the
COPY mix.exs mix.lock ./into two differentCOPYcommands, which one uses the cache and which does not?
I hope we can guess what’s wrong with this information 
