Update documentations for buildah support

Author: mkimuram

CHANGELOG.md

@@ -2,9 +2,12 @@
 
 ### Added
 
+- New option for [`operator-sdk build --image-builder`](https://github.com/operator-framework/operator-sdk/blob/master/doc/sdk-cli-reference.md#build), which can be used to specify which image builder to use. ([#1311](https://github.com/operator-framework/operator-sdk/pull/1311))
+
 ### Changed
 
 - When Helm operator projects are created, the SDK now generates RBAC rules in `deploy/role.yaml` based on the chart's default manifest. ([#1188](https://github.com/operator-framework/operator-sdk/pull/1188))
+- Renamed `--docker-build-args` option to `--image-build-args` option for `build` subcommand, because this option can now be shared with other image build tools than docker when `--image-builder` option is specified. ([#1311](https://github.com/operator-framework/operator-sdk/pull/1311))
 
 ### Deprecated
 

doc/sdk-cli-reference.md

@@ -16,7 +16,8 @@ Usage:
 * `--enable-tests` - enable in-cluster testing by adding test binary to the image
 * `--namespaced-manifest` string - path of namespaced resources manifest for tests (default "deploy/operator.yaml")
 * `--test-location` string - location of tests (default "./test/e2e")
-* `--docker-build-args` string - extra, optional docker build arguments as one string such as `"--build-arg https_proxy=$https_proxy"` (default "")
+* `--image-build-args` string - extra, optional image build arguments as one string such as `"--build-arg https_proxy=$https_proxy"` (default "")
+* `--image-builder` string - tool to build OCI images. One of: [docker, buildah] (default "docker")
 * `-h, --help` - help for build
 
 ### Use