Prevent from trying to build darwin images (#1279)

## Description

Defaults to linux for image platform. Errors out when explicitly provided darwin as container image platform.

## Type of change

<!--- Please check the type of change your PR introduces: --->
- [ ] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Test
- [x] 💻 CI/Deployment
- [ ] 🐹 Trivial/Minor

## Issue(s)

<!-- Can reference multiple issues. Use one of the following "magic words" - "closes, fixes" to auto-close the Github issue. -->
* fixes https://github.com/alcionai/corso/issues/1265

## Test Plan

<!-- How will this be tested prior to merging.-->
- [x] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E

build/build.sh

@@ -16,26 +16,21 @@ i386) GOARCH="386" ;;
 esac
 case "$(uname)" in
 Linux) GOOS="linux" ;;
-Darwin) GOOS="darwin" ;; # TODO: verify this
+Darwin) GOOS="darwin" ;;
 *) echo "Unknown OS" && exit 0 ;;
 esac
 
 PLATFORMS="$GOOS/$GOARCH" # default platform
 TAG="alcionai/corso"      # default image tag
 
-usage() {
-	echo "Usage: $(basename $0) <binary|image> [--platforms ...] [--tag ...]"
-	echo ""
-	echo "OPTIONS"
-	echo " -p|--platforms  Platforms to build for (default: $PLATFORMS)"
-    echo "                 Specify multiple platforms using ',' (eg: linux/amd64,darwin/arm)"
-	echo " -t|--tag        Tag for container image (default: $TAG)"
-}
-
 MODE="binary"
 case "$1" in
 binary) MODE="binary" && shift ;;
-image) MODE="image" && shift ;;
+image)
+	MODE="image"
+	shift
+	GOOS="linux" # darwin container images are not a thing
+	;;
 -h | --help) usage && exit 0 ;;
 *) usage && exit 1 ;;
 esac
@@ -49,6 +44,15 @@ while [ "$#" -gt 0 ]; do
 	shift
 done
 
+usage() {
+	echo "Usage: $(basename $0) binary | image [--platforms ...] [--tag ...]"
+	echo ""
+	echo "OPTIONS"
+	echo " -p | --platforms  Platforms to build for (default: $PLATFORMS)"
+	echo "                 Specify multiple platforms using ',' (eg: linux/amd64,darwin/arm)"
+	echo " -t | --tag        Tag for container image (default: $TAG)"
+}
+
 if [ "$MODE" == "binary" ]; then
 	mkdir -p ${CORSO_BUILD_CACHE} # prep env
 	for platform in ${PLATFORMS/,/ }; do
@@ -74,6 +78,10 @@ if [ "$MODE" == "binary" ]; then
 		echo Corso $platform binary available in "${ROOT}/bin/${GOOS}-${GOARCH}/${OUTFILE}"
 	done
 else
+	for platform in ${PLATFORMS/,/ }; do
+		echo "$platform" | grep -Eq "^darwin" &&
+			echo Cannot create darwin images "($platform)" && exit 1
+	done
 	echo Building "$TAG" image for "$PLATFORMS"
 	docker buildx build --tag ${TAG} \
 		--platform ${PLATFORMS} \