Is it possible to copy from another image inside a dockerfile, using a variable as parameter of the from flag?
What I would achieve is the following
ARG MY_VERSION
COPY --from=my-image:$MY_VERSION /src /dst
But i always get
invalid from flag value my-image:$MY_VERSION: invalid reference format
You can do what you want, but you need to have a FROM statement identifying the image you want to copy from. Something like this
ARG MY_VERSION
FROM my-image:$MY_VERSION as source
FROM scratch as final
COPY --from=source /src /dst
Replace scratch with the base image you want to use for your new image.
Here's an example to show that it works. Dockerfile:
ARG MY_VERSION
FROM ubuntu:$MY_VERSION as source
FROM alpine:latest
COPY --from=source /etc/os-release /
Build and run with
docker build --build-arg MY_VERSION=20.04 -t test .
docker run --rm test cat /os-release
The output shows
NAME="Ubuntu"
VERSION="20.04.2 LTS (Focal Fossa)"
ID=ubuntu
...
which shows that it has copied a file from the Ubuntu 20.04 image to an Alpine image.