Merge pull request juju#16390 from hpidcock/fix-podman

juju#16390

- Changes to prefer podman over docker.
- Podman doesn't use the default domain of docker.io in oci image references. This prefixes our docker org with docker.io to be compatible with podman and docker.

## QA steps

With only docker installed
- `make operator-image` should use docker
With podman installed (with or without docker also installed)
- `make operator-image` should use podman
- Images created should be prefixed with docker.io by default.

## Documentation changes

N/A

## Links

N/A

Makefile

@@ -453,8 +453,8 @@ check-deps:
 
 
 # CAAS related targets
-export OCI_BUILDER         ?= docker
-DOCKER_USERNAME            ?= jujusolutions
+export OCI_BUILDER         ?= $(shell (which podman 2>&1 > /dev/null && echo podman) || echo docker )
+DOCKER_USERNAME            ?= docker.io/jujusolutions
 DOCKER_BUILDX_CONTEXT      ?= juju-make
 DOCKER_STAGING_DIR         ?= ${BUILD_DIR}/docker-staging
 JUJUD_STAGING_DIR          ?= ${DOCKER_STAGING_DIR}/jujud-operator

make_functions.sh

@@ -12,7 +12,7 @@ JUJU_BUILD_NUMBER=${JUJU_BUILD_NUMBER:-}
 
 # Docker variables
 OCI_BUILDER=${OCI_BUILDER:-docker}
-DOCKER_USERNAME=${DOCKER_USERNAME:-jujusolutions}
+DOCKER_USERNAME=${DOCKER_USERNAME:-docker.io/jujusolutions}
 DOCKER_BUILDX_CONTEXT=${DOCKER_BUILDX_CONTEXT:-juju-make}
 DOCKER_STAGING_DIR="${BUILD_DIR}/docker-staging"
 DOCKER_BIN=${DOCKER_BIN:-$(which ${OCI_BUILDER} || true)}