Add Podman engine

[babykart]

PR related to #159

[edannenberg]

LGTM except for some minor stuff. 👍 No worries if you can't be bothered.

[edannenberg]

Squashed and merged. Thanks again! I'll leave this open until I get to update the docs for this. The 9999 ebuild already has a podman use flag now.

[babykart]

A last commit, sorry.
After reviewing in details the documentation of podman, commit and build have a default argument --format=oci.
I introduced a new variable/parameter (strictly like as build command) on commit phase, it's set as default --format=docker.

[babykart]

@edannenberg

With the last update on master, docker/podman commit fail with empty {DOCKER,PODMAN}_COMMIT_OPTS:

"docker commit" requires at least 1 and at most 2 arguments.

To fix it:

diff --git a/engine/docker.sh b/engine/docker.sh
index e81832c..6765493 100644
--- a/engine/docker.sh
+++ b/engine/docker.sh
@@ -229,7 +229,7 @@ function build_image() {
         _container_mount_portage='false'

         _status_msg="commit ${run_id} as image ${_current_namespace}/${builder_commit_id}:${IMAGE_TAG}"
-        pwrap 'nolog' "${DOCKER}" commit "${DOCKER_COMMIT_OPTS}" "${run_id}" "${_current_namespace}/${builder_commit_id}:${IMAGE_TAG}" \
+        pwrap 'nolog' "${DOCKER}" commit ${DOCKER_COMMIT_OPTS} "${run_id}" "${_current_namespace}/${builder_commit_id}:${IMAGE_TAG}" \
             || die "${_status_msg}"

[edannenberg]

My bad, somewhat blindly fixed the shellcheck error for that. Fixed.