WITH DOCKER breaks parallelization

[idelvall]

I would expect earthly +test-docker run the two builds in parallel, but that is not the case.

Notes:

• Commenting the FROM alpine line brings parallelization
• Moving FROM alpine outside of the target below VERSION 0.7, brings parallelization
• Using VERSION --no-use-registry-for-with-docker 0.7 brings parallelization
• The bug is reproducible both in CLI versions 0.7.23 and 0.8.3
• The bug is reproducible both with VERSION 0.7 and VERSION 0.8

Probably related to #2377

VERSION 0.7

a:
   FROM ubuntu
   RUN --no-cache bin/bash -c "for f in {1..10} ; do echo a ; sleep 1; done ;"

b:
   FROM ubuntu
   RUN --no-cache bin/bash -c "for f in {1..10} ; do echo b ; sleep 1; done ;"

docker-a:
   FROM ubuntu
   WITH DOCKER
     RUN --no-cache bin/bash -c "for f in {1..10} ; do echo a ; sleep 1; done ;"
   END

docker-b:
   FROM ubuntu
   WITH DOCKER
     RUN --no-cache bin/bash -c "for f in {1..10} ; do echo b ; sleep 1; done ;"
   END

# test properly runs +a and +b in parallel
test:
   FROM alpine
   BUILD +a
   BUILD +b

# test-docker is the failing one
test-docker:
   # Commenting the following line fixes parallelization
   FROM alpine
   BUILD +docker-a
   BUILD +docker-b

[vladaionescu]

Possible duplicate with #2377?