COPY --from command does not seem to support variables.

[mdgreenwald]

Expected behavior

Variable passed to COPY --from=${VARIABLE} /src /dest command would resolve.

Actual behavior

Variable is not resolved and instead generates error which indicates an invalid image name.
Service 'nginx' failed to build: invalid from flag value $name: invalid reference format

Information

• Full output of the diagnostics from "Diagnose & Feedback" in the menu
  50A907DF-6AD6-46D8-BB63-C833ED2F502F

• A reproducible case if this is a bug, Dockerfiles FTW

• Page URL if this is a docs issue or the name of a man page

Steps to reproduce the behavior

1. Copy zipped docker-compose and Dockerfile-nginx files into a folder.
2. run the command from that folder: docker-compose build --build-arg name="image_name" nginx

Output should be: ERROR: Service 'nginx' failed to build: invalid from flag value $name: invalid reference format

dockerfiles.zip

[ebriney]

Hi @mdgreenwald .
COPY --from must specify a multistage image alias
so you should use a Dockerfile like:

ARG name
FROM ${name} as base

FROM nginx:1.12.1-alpine

COPY --from=base /source/ /destination/

in order to get it works

[thaJeztah]

Correct, the --from option does not currently support variables; see moby/moby#34482 for more information.

I'll close this issue because this is not currently supported, thus not a bug, but feel free to comment after I closed

[lmapii]

@ebriney's solution does not seem to work for me: calling docker build -f api.Dockerfile -t thing-api:latest --build-arg arch=arm . for the following (snippet) Dockerfile

FROM build-base AS amd64
RUN make clean && make native

FROM build-base AS arm
RUN make clean && make xcompile TARGET_ARCH=arm TARGET_OS=linux

ARG arch=amd64
FROM ${arch} as build
# won't work either:
# COPY /thing-api /thing-api

FROM scratch
COPY --from=build /thing-api/api/src/main /main

EXPOSE 8080
CMD ["./main"]

will produce

Step 21/31 : FROM build-base AS amd64
 ---> bebf039c68b3
Step 22/31 : RUN make clean && make native
 ---> Using cache
 ---> 6b4b2a185443
Step 23/31 : FROM build-base AS arm
 ---> bebf039c68b3
Step 24/31 : RUN make clean && make xcompile TARGET_ARCH=arm TARGET_OS=linux
 ---> Using cache
 ---> 8ca0e7d07364
Step 25/31 : ARG arch=amd64
 ---> Using cache
 ---> 3b98282551aa
Step 26/31 : FROM ${arch} as build
 --->
Step 27/31 : COPY /thing-api /thing-api
COPY failed: stat /mnt/sda1/var/lib/docker/tmp/docker-builder093457154/thing-api: no such file or directory
make: *** [production] Error 1

However, if I replace --from=base with either --from=arm or --from=amd64 it'll work just fine.

[ebriney]

Hi @lmapii,
If you want to use it in FROM you must declare it before every FROM or it will be local to the previous one. It's bit strange I know.

# global arg
ARG arch=amd64

FROM build-base AS amd64
# local arg
ARG arch2=x86
RUN make clean && make native

FROM build-base AS arm
RUN make clean && make xcompile TARGET_ARCH=arm TARGET_OS=linux

FROM ${arch} as build

FROM scratch
COPY --from=build /thing-api/api/src/main /main

EXPOSE 8080
CMD ["./main"]

[lmapii]

Hello @ebriney - Thanks a lot for the feedback - I would have never guessed that and I tried quite some things ;) Now that you put it like that it kinda makes sense > and best of all it works. Very much appreciated !

[ebriney]

You're welcome. Same thing on my side, I read the docs and tried to make it works and finally asked to a colleague which gave me the (stupid) explanation.

[MartinMuzatko]

wow. this made me struggle for a few hours. thanks @mdgreenwald for reporting this.

[srobinson]

Same here. Does anyone know of a workaround?

COPY --from=image:${version}

[darwin]

This article describes a workaround (section 'Using build arguments in  — from'):
https://medium.com/@tonistiigi/advanced-multi-stage-build-patterns-6f741b852fae

Don't overlook:

Remember that any arguments used in FROM commands need to be defined before the first build stage.

[qhaas]

Found an edge case that impacts the (much appreciated) workaround by @ebriney, it occurs when one attempts to use an ARG to set the value of FROM in future build stages:

ARG DEPLOY_IMAGE=deploy-image
FROM ${DEPLOY_IMAGE} as deploy-image

ARG BASE_IMAGE=centos:7
FROM ${BASE_IMAGE}

BASE_IMAGE is part of deploy-image and thus is blank when used by the final FROM

UPDATE: The article posted by @darwin suggested moving the 2nd ARG before the first FROM, this works as expected, if a bit ugly:

ARG DEPLOY_IMAGE=deploy-image
ARG BASE_IMAGE=centos:7
FROM ${DEPLOY_IMAGE} as deploy-image

FROM ${BASE_IMAGE}

[jamshid]

Thanks for explanations I finally got it. All the ARGs, then FROMs, have to be at the beginning of the Dockerfile.

ARG REGISTRY_URL
ARG FOO_BRANCH
ARG BAR_BRANCH
FROM ${REGISTRY_URL}foo-src:${FOO_BRANCH} AS my-foo
FROM ${REGISTRY_URL}bar:${BAR_BRANCH} AS my-bar

FROM centos:8

COPY --from=my-foo /deliverables/*.rpm /deliverables/
COPY --from=my-bar /deliverables/*.rpm /deliverables/

RUN yum install -y /deliverables/*.rpm && yum clean all