Docker compose download link (#519)

## Description

Rebase of #305 (@MaxPeal) with reviewer requested changes applied.

notably, this variation on the original PR does not include `curl --progress`.

## Why is this needed

Fixes: #306

## How Has This Been Tested?
<!--- Please describe in detail how you tested your changes. -->
<!--- Include details of your testing environment, and the tests you ran to -->
<!--- see how your change affects other areas of the code, etc. -->


## How are existing users impacted? What migration steps/scripts do we need?

<!--- Fixes a bug, unblocks installation, removes a component of the stack etc -->
<!--- Requires a DB migration script, etc. -->


## Checklist:

I have:

- [ ] updated the documentation and/or roadmap (if required)
- [ ] added unit or e2e tests
- [ ] provided instructions on how to upgrade

setup.sh

@@ -257,7 +257,8 @@ setup_osie() (
 		pushd "$SCRATCH"
 
 		if [ -z "${TB_OSIE_TAR:-}" ]; then
-			curl -fsSL 'https://tinkerbell-oss.s3.amazonaws.com/osie-uploads/latest.tar.gz' -o ./osie.tar.gz
+			local OSIE_DOWNLOAD_LINK=${OSIE_DOWNLOAD_LINK:-"https://tinkerbell-oss.s3.amazonaws.com/osie-uploads/latest.tar.gz"} # If variable not set or null, use default.
+			curl -fsSL "${OSIE_DOWNLOAD_LINK}" -o ./osie.tar.gz
 			tar -zxf osie.tar.gz
 		else
 			if [ ! -f "$TB_OSIE_TAR" ]; then