ci: build a multi-arch image on each commit to main (#11544)

scripts/image_tag.sh

@@ -50,10 +50,16 @@ if [[ "$version" == "" ]]; then
 fi
 
 image="${CODER_IMAGE_BASE:-ghcr.io/coder/coder}"
-tag="v$version"
+
+# use CODER_IMAGE_TAG_PREFIX if set as a prefix for the tag
+tag_prefix="${CODER_IMAGE_TAG_PREFIX:-}"
+
+tag="${tag_prefix:+$tag_prefix-}v$version"
+
 if [[ "$version" == "latest" ]]; then
 	tag="latest"
 fi
+
 if [[ "$arch" != "" ]]; then
 	tag+="-$arch"
 fi