Add ARG support for multiple variables in one instruction #54
Description
The official documentation makes no mention of supporting multiple variables in a single ARG instruction (https://docs.docker.com/reference/dockerfile/#arg).
However, the docker command supports multiple variables. For example, the following does what one would expect:
FROM busybox
ARG a=1 b=2
RUN echo "a: $a"
RUN echo "b: $b"
i.e. it prints a: 1 and b: 2.
ARG can be used to declare variables without a default. Their value can then be set with --build-arg. The following declares two variables a and b:
FROM busybox
ARG a b
Where it gets weird is that it doesn't support comments but it identifies # as an ordinary string. The following declares 3 variables a, #, and b:
FROM busybox
ARG a # b
The following sets a to 2 even though it seems to be commented out:
$ cat bad-arg.dockerfile
FROM busybox
ARG a=1 # a=2
RUN echo "a: $a"
RUN echo "#: $#"
$ docker build -f bad-arg.dockerfile --build-arg '#=oh no' .
[+] Building 1.0s (7/7) FINISHED
=> [internal] load .dockerignore 0.0s
=> => transferring context: 2B 0.0s
=> [internal] load build definition from bad-arg.dockerfile 0.0s
=> => transferring dockerfile: 106B 0.0s
=> [internal] load metadata for docker.io/library/busybox:latest 0.0s
=> CACHED [1/3] FROM docker.io/library/busybox 0.0s
=> [2/3] RUN echo "a: 2" 0.4s
=> [3/3] RUN echo "#: oh no" 0.5s
=> exporting to image 0.0s
=> => exporting layers 0.0s
=> => writing image sha256:742fdc7edc6015ff3a4612019326b712f88571dc82cba 0.0s
tree-sitter-dockerfile doesn't currently support this syntax. We might want to support it even though its use should be discouraged.
(In Semgrep, we'd probably add a rule that forbids the use of # as a variable name and also maybe forbid multiple variables in a single ARG instruction.)