* Create a MultiPublisher
MultiPublisher mimics io.MultiWriter in that it will publish an image to
multiple publish.Interface implementations.
* Add publish.{Tarball,Layout}Publisher
This adds support for publishing in the tarball format and to an OCI
image layout.
The tarball format isn't great, yet. It only supports writing once
instead of appending.
* Consolidate options
These were spread all over the place for no reasons. Now all the
publisher related options are grouped together.
* Add options for tarball/layout
Adds --oci-layout-path, --tarball, and --push flags.
--push=false will disable the default behavior of publishing to a
registry.
* go mod vendor
* Add Close method to publish.Interface
This allows us to defer writing to the tarball until we've collected all
the images that have been published.
* Fix tests
* Add build.Limiter
You can limit the number of concurrent builds with -j (a la make).
The default value for this is GOMAXPROCS, which seems reasonable.
This hoists the publisher creation out of the loop for `ko publish` to
avoid needlessly resolving credentials multiple times.
This pulls the publisher creation outside of resolveFilesToWriter so we
can fail earlier if e.g. KO_DOCKER_REPO is unset, otherwise we start up
the kubectl goroutine and it would asynchronously fail with:
error: no objects passed to apply
