Update earthly/earthly Docker tag to v0.7.15 (#440)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

earthly.sh

@@ -1,3 +1,3 @@
 #!/bin/bash
 
-docker run --privileged -v /var/run/docker.sock:/var/run/docker.sock --rm -t -v "$(pwd):/workspace" -v earthly-tmp:/tmp/earthly:rw earthly/earthly:v0.7.9 --allow-privileged "$@"
+docker run --privileged -v /var/run/docker.sock:/var/run/docker.sock --rm -t -v "$(pwd):/workspace" -v earthly-tmp:/tmp/earthly:rw earthly/earthly:v0.7.15 --allow-privileged "$@"