User-data script has erroneous newline breaking shebang, incompatible with Cloud-init versions on AL2023

[AdamMcAdamson]

Summary

The resulting user-data script from cluster up has an erroneous newline which results in an invalid user-data script being sent to the EC2 instance. Older versions of Cloud-init ignored the issue (19-?, seen in AL2), but newer versions (22.2.2, seen in AL2023) correctly parse shell-scripts, causing the provided script to fail.

Description

Bug is here:

amazon-ecs-cli/ecs-cli/modules/cli/cluster/userdata/user_data.go

Lines 153 to 156 in 2791f9b

	joinClusterUserData := `
	#!/bin/bash
	echo ECS_CLUSTER=%s >> /etc/ecs/ecs.config
	`

Golang's MIME multipart writer already inserts a newline, leaving the resulting script with an erroneous newline at the start of the file:
https://github.com/golang/go/blob/master/src/mime/multipart/writer.go#L120

Expected Behavior

/var/lib/cloud/instance/scripts/part-001:

#!/bin/bash
echo ECS_CLUSTER=cluster-name >> /etc/ecs/ecs.config
...

Observed Behavior

/var/lib/cloud/instance/scripts/part-001:

#!/bin/bash
echo ECS_CLUSTER=cluster-name >> /etc/ecs/ecs.config
...

[AdamMcAdamson]

Fixed with #1168