Add Inference Tasks to CI

[nordzilla]

This pull request makes a few modifications to the taskcluster configurations for Docker. Each commit is separated into a logical, self-contained modification.

• Ensures that the IS_DOCKER environment variable is exported to 1 in all CI docker images.
• Modifies taskcluster naming conventions previously associated with all-pr to now be all-pr-pipeline.
• Introduces a new CI docker image, kind, and tasks for inference.

New tasks added to the CI Pipeline:

• @firefoxci-taskcluster inference-build Successful in 8m — FirefoxCI (pull_request)
• @firefoxci-taskcluster inference-build-wasm Successful in 13m — FirefoxCI (pull_request)
• @firefoxci-taskcluster inference-test Successful in 10m — FirefoxCI (pull_request)

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Note:

This is my first time working with taskcluster, taskgraph, etc. I tried to follow the existing code as an example, but I'm happy to make any further modifications based on best-practice recommendations.

The full pipeline training run has to run on this PR because I've modified the base Dockerfile, but ideally in the future only the inference-related tasks will run on inference-related PRs.

[nordzilla]

@bhearsum

Just retagging you for another quick glance now that I've responded to your feedback.