Variables that are defined in another build stage should not be suggested for completion

[rcjsuen]

Variables are scoped to a build stage. Thus, variables in one build stage shouldn't be suggested when we send a textDocument/completion request in a different build stage.

FROM scratch
ENV xyz=y
FROM alpine
RUN echo $xyz

$ docker build .
Sending build context to Docker daemon  1.695MB
Step 1/4 : FROM scratch
 --->
Step 2/4 : ENV xyz y
 ---> Running in 85a85f49228c
 ---> 4c491c4aebff
Removing intermediate container 85a85f49228c
Step 3/4 : FROM alpine
 ---> 3fd9065eaf02
Step 4/4 : RUN echo $xyz
 ---> Running in 854c6e67692b

 ---> 75e57160efb1
Removing intermediate container 854c6e67692b
Successfully built 75e57160efb1

FROM scratch
ARG xyz=y
FROM alpine
RUN echo $xyz

$ docker build .
Sending build context to Docker daemon  1.695MB
Step 1/4 : FROM scratch
 --->
Step 2/4 : ARG xyz=y
 ---> Running in ce3339cc0e85
 ---> 8789ac237b75
Removing intermediate container ce3339cc0e85
Step 3/4 : FROM alpine
 ---> 3fd9065eaf02
Step 4/4 : RUN echo $xyz
 ---> Using cache
 ---> 75e57160efb1
Successfully built 75e57160efb1

[rcjsuen]

This isn't a bug in the language server. The parser isn't returning the right stuff. See rcjsuen/dockerfile-ast#15.