How does Docker determine the need to update or reset cached data?

Docker uses a layer caching mechanism for images to speed up the build process. When executing instructions in a Dockerfile, Docker checks whether the context or the instruction itself has changed since the last build. If the instruction and its context (e.g., the contents of copied files) have not changed, Docker uses the cached layer, avoiding re-execution of the command.

Key points affecting cache invalidation:

• Changes in the contents of files that are copied or used in instructions (e.g., COPY, ADD).
• Changes in the instruction itself in the Dockerfile.
• Changes in environment variables or arguments that influence the command.

If Docker detects that the instruction or its context has changed, it invalidates the cache for that instruction and subsequent ones.

Example:

FROM python:3.9
COPY requirements.txt /app/
RUN pip install -r /app/requirements.txt
COPY . /app
CMD ["python", "/app/app.py"]

If the requirements.txt file does not change, Docker will not re-run pip install, using the cache. But if requirements.txt changes, the cache for that step and subsequent steps will be invalidated.