design for multi-arch build and windows build - add local build to tar

Signed-off-by: Lyndon-Li <[email protected]>

design/multiple-arch-build-with-windows.md

@@ -17,8 +17,9 @@ At present, Velero by default build images locally, or no image or manifest is p
 
 ## Local Build
 
-For local build, the images are only built for the platform (`<os>/<arch>`) as same as the building env. E.g., when building from linux-amd64 env, a single manifest of linux-amd64 is created regardless how the input parameters are configured.  
-For local build, the `--output` parameter for `docker buildx build` is always `docker`.  
+For local build, two values of `--output` parameter for `docker buildx build` are supported:
+- `docker`: a docker format image is built, but the image is only built for the platform (`<os>/<arch>`) as same as the building env. E.g., when building from linux-amd64 env, a single manifest of linux-amd64 is created regardless how the input parameters are configured.  
+- `tar`: one or more images are built as tarballs according to the input platform (`<os>/<arch>`) parameters. Specifically, one tarball is generated for each platform. The build process is the same with the `Build Separate Manifests` of `Push Build` as detailed below. Merely, the `--output` parameter diffs, as `type=tar;dest=<tarball generated path>`. The tarball is generated to the `_output` folder and named with the platform info, e.g., `_output/velero-main-linux-amd64.tar`.  
 
 ## Push Build
 
@@ -46,7 +47,7 @@ The created manifest will be pushed to registry by command `docker manifest push
 ## Input Parameters
 
 Below are the input parameters that are configurable to meet different build purposes during Dev and release cycle:
-- BUILD_PUSH: whether to make a push build. Default value is `false`  
+- BUILD_OUTPUT_TYPE: the type of output for the build, i.e., `docker`, `tar`, `registry`, while `docker` and `tar` is for local build; `registry` means push build. Default value is `docker`  
 - BUILD_OS: which types of OS should be built for. Multiple values are accepted, e.g., `linux,windows`. Default value is `linux`  
 - BUILD_ARCH: which types of architecture should be built for. Multiple values are accepted, e.g., `amd64,arm64`. Default value is `amd64`  
 - BUILD_TAG_GCR: whether to build and push GCR images. GCR images are not likely required during Dev cycle, so the default value is `false`  
@@ -56,8 +57,9 @@ Below are the input parameters that are configurable to meet different build pur
 Windows container images vary from Windows OS versions, e.g., `ltsc2022` for Windows server 2022 and `1809` for Windows server 2019. Images for different OS versions should be built separately.  
 Therefore, separate build targets are added for each OS version, like `container-windows-%`.  
 For the same reason, a new input parameter is added, `BUILD_WINDOWS_VERSION`. The default value is `ltsc2022`. Windows server 2022 is the only base image we will deliver officially, Windows server 2019 is not supported. In future, we may need to support Windows server 2025 base image.  
+For local build to tar, the Windows OS version is also added to the name of the tarball, e.g., `_output/velero-main-windows-ltsc2022-amd64.tar`.  
 
-At present, Windows container image only supports `amd64` as the architecture, so `BUILD_BUILD_ARCH` is ignored for Windows.  
+At present, Windows container image only supports `amd64` as the architecture, so `BUILD_ARCH` is ignored for Windows.  
 
 The Windows manifests need to be annotated with os type, arch, and os version. This will be done through `docker manifest annotate` command.  
 
@@ -67,16 +69,37 @@ In order to use the images, the manifest list's tag should be provided to `veler
 
 ## Build Samples
 
-**Local build**
+**Local build to docker**
 ```
 make container
 ```
-The built image could be listed by `docker image ls`.
+The built image could be listed by `docker image ls`.  
+
+**Local build for linux-amd64 and windows-amd64 to tar**
+```
+BUILDX_OUTPUT_TYPE=tar BUILD_OS=linux,windows make container
+```
+Under `_output` directory, below files are generated:  
+```
+velero-main-linux-amd64.tar
+velero-main-windows-ltsc2022-amd64.tar
+``` 
+
+**Local build for linux-amd64, linux-arm64 and windows-amd64 to tar**
+```
+BUILDX_OUTPUT_TYPE=tar BUILD_OS=linux,windows BUILD_ARCH=amd64,arm64 make container
+```
+Under `_output` directory, below files are generated:  
+```
+velero-main-linux-amd64.tar
+velero-main-linux-arm64.tar
+velero-main-windows-ltsc2022-amd64.tar
+```
 
 **Push build for linux-amd64 and windows-amd64**  
 Prerequisite: login to registry, e.g., through `docker login`  
 ```
-BUILD_PUSH=true REGISTRY=<registry> BUILD_BUILD_OS=linux,windows make container
+BUILDX_OUTPUT_TYPE=registry REGISTRY=<registry> BUILD_OS=linux,windows make container
 ```
 Nothing is available locally, in the registry 3 tags are available:
 ```
@@ -88,7 +111,7 @@ velero/velero:main-linux-amd64
 **Push build for linux-amd64, linux-arm64 and windows-amd64**  
 Prerequisite: login to registry, e.g., through `docker login` 
 ```
-BUILD_PUSH=true REGISTRY=<registry> BUILD_BUILD_OS=linux,windows BUILD_BUILD_ARCH=amd64,arm64 make container
+BUILDX_OUTPUT_TYPE=registry REGISTRY=<registry> BUILD_OS=linux,windows BUILD_ARCH=amd64,arm64 make container
 ```
 Nothing is available locally, in the registry 4 tags are available:
 ```
@@ -97,6 +120,3 @@ velero/velero:main-windows-ltsc2022-amd64
 velero/velero:main-linux-amd64
 velero/velero:main-linux-arm64
 ```
-
-
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