**The Bug:**
You may have seen log lines like this when running 0.9.0-dev:
```
... client.alloc_runner.task_runner: some environment variables not available for rendering: ... keys="attr.driver.docker.volumes.enabled, attr.driver.docker.version, attr.driver.docker.bridge_ip, attr.driver.qemu.version"
```
Not only should we not be erroring on builtin driver attributes, but the
results were nondeterministic due to map iteration order!
The root cause is that we have an old root attribute for all drivers
like:
```
attr.driver.docker = "1"
```
When attributes were opaque variable names it was fine to also have
"nested" attributes like:
```
attr.driver.docker.version = "1.2.3"
```
However in the HCLv2 world the variable names are no longer opaque: they
form an object tree. The `docker` object can no longer both hold a value
(`"1"`) *and* nested attributes (`version = "1.2.3"`).
**The Fix:**
Since the old `attr.driver.<name> = "1"` attribues are useless for task
config interpolation, create a new precedence rule for creating the task
config evaluation context:
*Maps take precedence over primitives.*
This means `attr.driver.docker.version` will always take precedence over
`attr.driver.docker`. The results are determinstic and give users access
to the more useful metadata.
I made this a general precedence rule instead of special-casing driver
attrs because it seemed like better default behavior than spamming
WARNings to logs that were likely unactionable by users.
