Arguments not specified at the top of the Dockerfile are ignored

[BorekZnovustvoritel]

How to reproduce

Suppose we have a file called Dockerfile:

ARG foo
FROM $foo

If we run the command dockerfile-json --build-arg=foo=alpine Dockerfile, the tool correctly interprets that the base image to use is alpine.

However when we change the contents slightly:

FROM ubuntu
# ...
ARG foo
FROM $foo

then the tool will interpret the last step as {... "From": {"Image": ""}, ...} even with the passed --build-arg, which breaks base image parsing.

Expected behavior

The argument foo should be interpreted in the same way in both of these cases. I know this can be worked around by modifying the Dockerfile (pushing all ARGs to the top of the Dockerfile), but both of these formats are valid and only one of them is supported by dockerfile-json.

[chmeliik]

There are two kinds of ARGs:

• Meta args - those defined before the first FROM instruction. These can be referenced in FROM instructions but not inside build stages
• Local args - defined after a FROM instruction. They can be referenced within the same build stage but not in FROM instructions or other stages.

See also https://docs.docker.com/reference/dockerfile/#understand-how-arg-and-from-interact

So the behavior is correct, in the second Dockerfile the second base image should expand to an empty string.

That being said, dockerfile-json doesn't handle local args/envs correctly. I'm working on that in konflux-ci#7