Add .ko.yaml to base ko container image on golang:1.17 (#533)

* Add .ko.yaml to base ko container image on golang:1.17

Ignore this in e2e tests, since even though golang:1.17 provides a
Windows base image, ko's base image platform selection doesn't take
osversion into account. The e2e tests don't exercise golang usage at
all, so it shouldn't matter.

* Update README

.github/workflows/e2e.yaml

@@ -33,6 +33,9 @@ jobs:
         eval $(go env | sed -r 's/^(set )?(\w+)=("?)(.*)\3$/\2="\4"/gm')
 
         if [[ "${{ matrix.platform }}" == "windows-latest" ]]; then
+          # Always use the nanoserver base image, which matches the Windows
+          # version used by the GitHub Actions Windows runner.
+          rm .ko.yaml
           export KO_DEFAULTBASEIMAGE=mcr.microsoft.com/windows/nanoserver:1809
         fi
 

.ko.yaml

@@ -0,0 +1,2 @@
+baseImageOverrides:
+  github.com/google/ko: golang:1.17

README.md

@@ -466,9 +466,11 @@ You can try out building a Windows container image by [setting the base image](#
 For example, to build a Windows container image for `ko`, from within this repo:
 
 ```
-KO_DEFAULTBASEIMAGE=mcr.microsoft.com/windows/nanoserver:1809 ko publish ./ --platform=windows/amd64
+ko publish ./ --platform=windows/amd64
 ```
 
+This works because the `ko` image is configured in [`.ko.yaml`](./.ko.yaml) to be based on a `golang` base image, which provides platform-specific images for both Linux and Windows.
+
 ### Known issues 🐛
 
 - Symlinks in `kodata` are ignored when building Windows images; only regular files and directories will be included in the Windows image.