fix(dev): connect agent containers to dev network

[simonjcarr]

Summary

• add a systemd-capable local Ubuntu agent test container helper
• seed dev-stack agent container defaults in .dev/dev.env
• connect local agent test containers to the dev-stack Docker network using internal service names
• make installer bootstrap failures fail loudly instead of being hidden by a pipe
• add image healthcheck and set a non-root default user while explicitly running the systemd installer test container as root

Validation

• bash -n dev-stack.sh
• bash -n deploy/scripts/create-agent-dev-container.sh
• bash -n deploy/scripts/test-dev-stack.sh
• bash -n deploy/scripts/test-create-agent-dev-container.sh
• deploy/scripts/test-dev-stack.sh
• deploy/scripts/test-create-agent-dev-container.sh

Replaces #1431 and #1430.