Multi stage build fails when referencing a previous stage with ARG

[erandagan]

TLDR FROM <previous-stage-alias>-${SOME_ARG} fails Kaniko.

This issue presents a special case of #233 where the stage alias contains a reference to an ARG, a pattern supported by docker & BuildKit.

Actual behavior
A handy feature of Multistage builds is the ability to dynamically select a base stage in later stages:

ARG VARIANT

FROM scratch AS base
# Shared stuff over here...

FROM base AS base-dev
# Some dev only stuff here, like installing debugging tools.

FROM base AS base-prod
# Some prod only stuff here, like running Release builds or installing prod-only dependencies.

FROM base-${VARIANT}
# Final image dynamically selected. 
# Some final processing steps here, like ENTRYPOINT.

In the above example, when VARIANT is set to dev, the final image will inherit from base-dev. This pattern works great on stock Docker & BuildKit (which even skips unused branches, like base-prod, to speed up the build further). However, on Kaniko, the above build will fail

VARIANT=dev:

INFO[0003] Downloading base image base-dev              
2019/08/22 11:32:08 No matching credentials were found, falling back on anonymous
error building image: UNAUTHORIZED: authentication required; [map[Name:library/base-dev Action:pull Type:repository Class:]]

But, if we replace FROM base-${VARIANT} with a hardcoded FROM base-dev, everything is resolved correctly, though it renders the dynamic base picking useless.

So, it seems that Kaniko doesn't properly resolve stage aliases when they contain an ARG expression.

Expected behavior
Kaniko should correctly handle FROM <previous-stage-name>-${SOME_ARG} instructions.

To Reproduce
Steps to reproduce the behavior:

1. Write a Dockerfile similar to the one above
2. Attempt to build it with Kaniko (should fail)
3. Attempt to build it using docker (should succeed)

Additional Information

• Dockerfile
  As presented in the example above.
• Build Context
  --build-arg VARIANT=dev
• Kaniko Image (fully qualified with digest)
  gcr.io/kaniko-project/executor@sha256:78d44ec4e9cb5545d7f85c1924695c89503ded86a59f92c7ae658afa3cff5400

[tejal29]

@erandagan Currently we do not support evaluating args.
We do support resolving env varaibles in the stage name
https://github.com/GoogleContainerTools/kaniko/blob/master/pkg/dockerfile/dockerfile.go#L67

We need to do something similar something similar for Args.

Feel free to take a stab at it.