Proposal: Global ARGs

[vladaionescu]

Background

@ido-namely reports on Gitter:

I have different targets building different images, and I'd like them all to use the same docker registry address.

I was hoping that ARG can be shared

ARG TST="hello world"
FROM golang:1.13-alpine3.11

test-target:
    ARG TST
    RUN echo $TST

Proposal

• All args declared in the +base target are "global" ARGs. Meaning that they are passed throughout the other targets even if they don't inherit from +base. So the following should print "hello world":

ARG TST="hello world"
FROM golang:1.13-alpine3.11

test-target:
    RUN echo $TST

Possible future extension

Could perhaps prevent this behavior if needed with a ARG --local TST="hello world"

Alternatives considered

• A possibility might be that ARGs declared in the +base target are global and shared with the rest of the targets, if there is no FROM and they inherit automatically from +base. This would be inconsistent when chaining targets.
• Another option could be to always propagate build args too on a FROM. This may be deviating too much from Dockerfile behavior.
• Yet another option is to have a special arg ARG --global for such situations.

[vladaionescu]

In v0.4.0