Task file vars interpolation does not work for container_limits element

Summary

Having a pipeline with a task file whose container_limits are hard coded is working for us.
When we try to make the container limits configurable, the build breaks with an unparseble task file.

Working:

---
platform: linux

container_limits:
  cpu: 32696
  memory: "5gb"

image_resource:
  type: docker-image
  source:
    repository: adoptopenjdk14-dind
    tag: latest
    insecure_registries: ['any.registry.com']
    username: ((concourse_user))
    password: ((concourse_password))

[…]

Not working:

---
platform: linux

container_limits:
  cpu: 32696
  # configured via vars: in the pipeline
  memory: ((container_mem_limit)) 

image_resource:
  type: docker-image
  source:
    repository: adoptopenjdk14-dind
    tag: latest
    insecure_registries: ['any.registry.com']
    username: ((concourse_user))
    password: ((concourse_password))

[…]

Expected results

The container limits in a task file should work from configured vars: in the pipeline.

Actual results

Concourse cannot launch the task complaining that for example the memory limit cannot be parsed for the task file.

Additional context

Might be related to #5753

Triaging info

• Concourse version: 6.2
• Did this used to work? => Unsure, tested it the first time with 6.2

Start by reproducing the failure with the working and variable-based task YAML, then trace the task-file container_limits parsing and vars interpolation entry points in the Concourse codebase. Done means a configured pipeline variable supplies container_limits.memory without an unparseable task-file error, with the behavior verified for the provided example.