FROM ubuntu:latest
VOLUME /myVol
RUN echo "Hello World" > /myVol/message.txt
CMD cat /myVol/message.txt

When I run the container (after a successful image build), I get "Container cannot find file in volume- "No such file or directory". When I open an interactive session to the container, I see the volume, but no file (from the RUN echo..) in it. Why?

CodePudding user response：

The most visible effect of VOLUME is to prevent any subsequent RUN command from modifying the VOLUME directory; this is mentioned under "Notes about specifying volumes" in the linked documentation.

For most purposes, you don't need VOLUME. In particular you do not need to declare a directory a VOLUME in the Dockerfile just because you expect an operator will eventually mount something there; when a container is run Docker volumes and host directories can be mounted on any container path regardless of whether or not it's a VOLUME.

You should be able to safely delete the VOLUME line with no consequences, and the file you're creating will actually exist.