Add Podman compatibility to vox:build task

Prior to this commit, running the `vox:build` task via the
`podman-docker` shim would fail as `podman ls --format json` prints
an empty array, `[]`, when no containers match `--filter`. This differs
from `docker ls` behavior of printing nothing to STDOUT. This extra
output resulted in the `container_exists` function returning a false
positive, which caused the task to fail when trying to run `podman stop`
on containers that didn't exist.

This commit changes the command to use `--format '{{json .ID}}'` as
this produces the same behavior from both `podman` and `docker`:
no STDOUT when nothing matches, and a quoted string containing the
container ID when there is a match.

Signed-off-by: Charlie Sharpsteen <charlie@overlookinfratech.com>

Author: Sharpie

tasks/build.rake

@@ -54,7 +54,7 @@ def image_exists
 end
 
 def container_exists
-  !`docker container ls --all --filter 'name=#{@container}' --format json`.strip.empty?
+  !`docker container ls --all --filter 'name=#{@container}' --format '{{json .ID}}'`.strip.empty?
 end
 
 def teardown