executors: Add support for private docker registries

[eseliger]

This PR adds support for private registries to executors, so for code intel auto-indexing and server-side batch changes.
It does so by introducing a new environment variable and a magic secret DOCKER_AUTH_CONFIG that can be set to authenticate to a protected docker registry. See the inline docs change for how this works exactly. I've also added storybooks coverage for the new components, so make sure to check out the UI review check.

There are two pending TODOs left but they shouldn't affect the code much so opening up for review now to get eyes on this!

Pending TODOs:

• I need to roll this out to a dogfood cluster and test in firecracker (not on Mac 🙃)

Sister PRs:

• Add support for private registries terraform-google-executors#74
• Add support for private registries terraform-aws-executors#57

Closes https://github.com/sourcegraph/sourcegraph/issues/40479

Test plan

Verified all documented variants work locally and will test before merge on a dogfood instance to verify firecracker works as well.

[sg-e2e-regression-test-bob]

Bundle size report 📦

Initial size	Total size	Async size	Modules
0.07% (+1.96 kb)	0.02% (+3.02 kb)	0.01% (+1.06 kb)	0.00% (0)

Look at the Statoscope report for a full comparison between the commits 35e171e and 8c996a7 or learn more.

Open explanation

• Initial size is the size of the initial bundle (the one that is loaded when you open the page)
• Total size is the size of the initial bundle + all the async loaded chunks
• Async size is the size of all the async loaded chunks
• Modules is the number of modules in the initial bundle

[sourcegraph-bot]

Codenotify: Notifying subscribers in CODENOTIFY files for diff 8c996a7...35e171e.

Notify	File(s)
@courier-new	client/web/src/enterprise/batches/list/BatchChangesChangelogAlert.tsx
@efritz	client/web/src/enterprise/executors/secrets/AddSecretModal.story.tsx client/web/src/enterprise/executors/secrets/AddSecretModal.tsx client/web/src/enterprise/executors/secrets/ExecutorSecretNode.tsx client/web/src/enterprise/executors/secrets/ExecutorSecretsListPage.story.tsx client/web/src/enterprise/executors/secrets/UpdateSecretModal.story.tsx client/web/src/enterprise/executors/secrets/UpdateSecretModal.tsx enterprise/cmd/executor/internal/command/docker.go enterprise/cmd/executor/internal/command/docker_test.go enterprise/cmd/executor/internal/command/firecracker.go enterprise/cmd/executor/internal/command/firecracker_test.go enterprise/cmd/executor/internal/command/runner.go enterprise/cmd/executor/internal/config/config.go enterprise/cmd/executor/internal/run/testvm.go enterprise/cmd/executor/internal/run/util.go enterprise/cmd/executor/internal/worker/handler.go enterprise/cmd/executor/internal/worker/worker.go enterprise/cmd/frontend/internal/executorqueue/queues/batches/transform.go enterprise/cmd/frontend/internal/executorqueue/queues/batches/transform_test.go enterprise/cmd/frontend/internal/executorqueue/queues/codeintel/queue.go enterprise/cmd/frontend/internal/executorqueue/queues/codeintel/transform.go enterprise/cmd/frontend/internal/executorqueue/queues/codeintel/transform_test.go enterprise/internal/executor/client_types.go enterprise/internal/executor/client_types_test.go
@sourcegraph/delivery	doc/admin/deploy_executors.md