Add ImageBuilder container build instructions (#1448)

Author: lbussell

README.md

@@ -1,7 +1,9 @@
-# docker-tools
-This is a repo to house some common tools for use in the various .NET Docker repos. 
+# Docker Tools
+
+This is a repo to house some common tools for use in the various .NET Docker repos.
+
+## Image Builder
 
-# Image Builder
 A tool used to build and publish Docker images.
 
 The Image Builder tool can be acquired via a Docker image available at [mcr.microsoft.com/dotnet-buildtools/image-builder](https://mcr.microsoft.com/v2/dotnet-buildtools/image-builder/tags/list) or built from source via the [build script](./src/Microsoft.DotNet.ImageBuilder/build.ps1).
@@ -14,4 +16,4 @@ The full list of supported commands can be seen by running the tool.
 
 The list of support command options can be seen by specifying the `-h` command option.  The following illustrates how to list the build options.
 
-- Linux container environment: `docker run -it --rm -v /var/run/docker.sock:/var/run/docker.sock mcr.microsoft.com/dotnet-buildtools/image-builder:debian-20190223173930 build -h` 
+- Linux container environment: `docker run -it --rm -v /var/run/docker.sock:/var/run/docker.sock mcr.microsoft.com/dotnet-buildtools/image-builder:debian-20190223173930 build -h`

src/Microsoft.DotNet.ImageBuilder/README.md

@@ -0,0 +1,39 @@
+# ImageBuilder
+
+ImageBuilder is a tool used to build and publish Docker images.
+
+## Building the ImageBuilder container image
+
+All commands are relative to the root of the repo.
+
+### Build a single-platform image
+
+Using Linux or Windows, simply run the build script:
+
+```pwsh
+# From src/Microsoft.DotNet.ImageBuilder
+pwsh -f build.ps1
+
+# From the root of the repo
+pwsh -wd ./src/Microsoft.DotNet.ImageBuilder/ -f src/Microsoft.DotNet.ImageBuilder/build.ps1
+```
+
+### Build a multi-arch Linux image
+
+If you don't need to test on Windows, this is the easiest way to create a multi-arch manifest list.
+
+```pwsh
+# Build the image. Choose one or both platforms, and optionally push to a registry or load the image locally.
+docker buildx build [--push,--load] --platform [linux/amd64,linux/arm64] -t "${REPO}:${TAG}" -f .\src\Microsoft.DotNet.ImageBuilder\Dockerfile.linux .\src\Microsoft.DotNet.ImageBuilder\
+```
+
+### Create a multi-platform manifest list
+
+First, build and push Linux and Windows images separately.
+Gather the specific digests for the images you want to put into one manifest list.
+Then, create the manifest list and push it:
+
+```pwsh
+docker manifest create "${REPO}:${TAG}" "${REPO}@sha256:abcde12345" "${REPO}@sha256:fghij67890"
+docker manifest push "${REPO}:${TAG}"
+```