ARG is evaluated twice (once under +base, once under +target)

[alexcb]

foo:
    FROM alpine
    ARG myvar
    WORKDIR /test
    RUN echo "myvar is $myvar\n; dir contents is $(ls -la)"

run:
    FROM alpine
    RUN echo bar > data
    BUILD +foo --myvar="$(cat data)"

under 0.5.24, it worked:

$ earthly-v0.5.24 --buildkit-volume-name=five-two-four +run
           buildkitd | Starting buildkit daemon as a docker container (earthly-buildkitd)...
           buildkitd | ...Done
              alpine | --> Load metadata linux/amd64
                +run | --> FROM alpine
                +run | [██████████] resolve docker.io/library/alpine@sha256:21a3deaa0d32a8057914f36584b5288d2e5ecc984380bc0118285c70fa8c9300 ... 100%
                +run | [██████████] sha256:59bf1c3509f33515622619af21ed55bbe26d24913cedbca106468a5fb37a50c3 ... 100%
                +run | [██████████] extracting sha256:59bf1c3509f33515622619af21ed55bbe26d24913cedbca106468a5fb37a50c3 ... 100%
                +run | --> RUN echo bar > data
                +run | --> ARG myvar = RUN $(cat data)
                +foo | myvar=bar
                +foo | --> WORKDIR /test
                +foo | myvar=bar
                +foo | --> RUN echo "myvar is $myvar\n; dir contents is $(ls -la)"
                +foo | myvar is bar\n; dir contents is total 8
                +foo | drwxr-xr-x    2 root     root          4096 Nov 26 19:04 .
                +foo | drwxr-xr-x    1 root     root          4096 Nov 26 19:04 ..

However under 0.6.0, it now fails:

 1. Init 🚀
————————————————————————————————————————————————————————————————————————————————

           buildkitd | Found buildkit daemon as docker container (earthly-buildkitd)
           buildkitd | Updated image available. Restarting buildkit daemon...
           buildkitd | ...Done


 2. Build 🔧
————————————————————————————————————————————————————————————————————————————————

Error: build target: build main: bkClient.Build: failed to solve: parse build args: parse build arg myvar=$(cat data): the first command has to be FROM, FROM DOCKERFILE, LOCALLY, ARG, BUILD or IMPORT

If however we introduce a FROM ... under the base, like:

FROM alpine

foo:
    FROM alpine
    RUN echo foo-data /data
    ARG myvar
    RUN echo "myvar is $myvar\n"

run:
    FROM alpine
    RUN echo run-data > /data
    BUILD +foo --myvar="$(cat /data)"

this works, and produces:

 1. Init 🚀
————————————————————————————————————————————————————————————————————————————————

           buildkitd | Found buildkit daemon as docker container (earthly-buildkitd)


 2. Build 🔧
————————————————————————————————————————————————————————————————————————————————

              alpine | --> Load metadata linux/amd64
               +base | --> FROM alpine
               +base | [██████████] 100% resolve docker.io/library/alpine@sha256:21a3deaa0d32a8057914f36584b5288d2e5ecc984380bc0118285c70fa8c9300
               +base | [██████████] 100% sha256:59bf1c3509f33515622619af21ed55bbe26d24913cedbca106468a5fb37a50c3
               +base | [██████████] 100% extracting sha256:59bf1c3509f33515622619af21ed55bbe26d24913cedbca106468a5fb37a50c3
               +base | --> RUN echo base-data /data
               +base | base-data /data
                +run | --> ARG myvar = RUN $(cat /data)
                +run | cat: can't open '/data': No such file or directory
                +run | --> RUN echo run-data > /data
                +run | --> ARG myvar = RUN $(cat /data)
                +foo | --> RUN echo foo-data /data
                +foo | foo-data /data
                +foo | myvar=run-data
                +foo | --> RUN echo "myvar is $myvar\n"
                +foo | myvar is run-data\n
              output | --> exporting outputs

It's interesting to note: +run | cat: can't open '/data': No such file or directory indicates it's being evaluated under +base; however, when we make it to +foo | --> RUN echo "myvar is $myvar\n", it correctly prints out +foo | myvar is run-data\n. It looks like this is being (incorrectly) evaluated twice: once under +base and another time under +run.