[SC-30890] Update README on how to test locally

Adds instructions on how to test locally, and adds an option to
`./build-image` for building locally.

README.md

@@ -3,32 +3,11 @@
 This repository has a workflow defined that will build and push amd64 and arm64
 images to Github Container Registry.
 
-# How to build this using docker
+# How to build and test locally
 
-* Use `./build-image.sh push-arch-tag` on your machine to build a local image
+* Use `./build-image.sh build-local` on your machine to build a local image
   for testing. As you make changes locally the images will be tagged with
   `-uncommitted` on the end rather than the commit SHA.
 
 * Update the base tag in `./build-image.sh` as necessary to reflect changes
   in the base image or GHC version included in the image.
-
-* On an AMD64 machine:
-
-```
-./build-image.sh build-arch-tag
-./build-image.sh push-arch-tag
-```
-
-* On an ARM64 machine:
-
-```
-./build-image.sh build-arch-tag
-./build-image.sh push-arch-tag
-```
-
-* On either machine:
-
-```
-./build-image.sh push-manifest
-```
-

build-image.sh

@@ -40,6 +40,10 @@ esac
 COMMAND=$1
 
 case $COMMAND in
+  build-local)
+    echo "Building $ARCH_TAG"
+    docker build -t $ARCH_TAG .
+    ;;
   build-and-push-arch-tag)
     echo "Building $ARCH_TAG"
     docker buildx build . \