add test/run/in-docker

[hans-d]

what

• add test/run/in-docker target
• adds AWS_ENDPOINT_URL

why

• use generic target in test harness, vs specific per repositpry
• allows for bulk update of eg env vars to pass on
• still allowing repo sepcific setup
• AWS_ENDPOINT_URL allows for injection for eg localstacl

notes

• not fully clear how the full harness structure works, esp regarding variable inheritance/overrides.
• likely we can move the two prefixed vars to the top of the makefile
• there is already a terraform_version var with a predefined value in the test/terraform section, that one is different from the default in the test/src (in the calling repo), hence opted for a new var.