Backwards compatability test for WITH DOCKER

[alexcb]

Introduces a new test for validating earthly v0.8.0 can work with the current version of earthly-buildkitd.

Fixes #4128

[alexcb]

You can also run this locally against any versions, e.g. this reproduces the error:

earthly=earthly-v0.8.10 crustly=earthly-v0.8.9 ./backwards-compatability.sh

which produces:

          buildkitd | Found buildkit daemon as docker container (earthly-integration-buildkitd)
           buildkitd | Updated image available. But update was inhibited.
           buildkitd | Warning: Buildkit version (v0.8.10) is different from Earthly version (v0.8.9)

...

================================== ❌ FAILURE ===================================

././t/with-docker+empty-test *failed* | Repeating the failure error...
././t/with-docker+empty-test *failed* | --> WITH DOCKER RUN --privileged echo "dummy"
././t/with-docker+empty-test *failed* | /var/earthly/dockerd-wrapper.sh: line 80: EARTHLY_DOCKERD_CACHE_DATA: parameter not set
././t/with-docker+empty-test *failed* | ERROR ../../tests/with-docker/Earthfile line 23:8
././t/with-docker+empty-test *failed* |       The command
././t/with-docker+empty-test *failed* |           WITH DOCKER RUN --privileged echo "dummy"
././t/with-docker+empty-test *failed* |       did not complete successfully. Exit code 2
././t/with-docker+empty-test | unlazy force execution: process "/bin/sh -c EARTHLY_DOCKERD_DATA_ROOT=\"/var/earthly/dind/69b63098c23beedcaa2fbd5c887864c5d8a8f4d02f64752e7394e5dc20528d14\" EARTHLY_DOCKER_LOAD_FILES=\"\" EARTHLY_IMAGES_WITH_DIGESTS=\"\" EARTHLY_START_COMPOSE=\"false\" EARTHLY_COMPOSE_FILES=\"\" EARTHLY_COMPOSE_SERVICES=\"\" DIND_IMAGE=earthly/dind:alpine-3.19-docker-25.0.5-r0 OTEL_TRACES_EXPORTER=none PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin /var/earthly/dockerd-wrapper.sh execute /usr/bin/earth_debugger /bin/sh -c 'echo \"dummy\"'" did not complete successfully: exit code: 2

[mikejholly]

😂