ci: build the desktop template on every cluster (foxtrot, juliett, tango)

[devin-ai-integration]

Summary

The desktop template is now built on every production cluster — foxtrot, juliett and tango — instead of foxtrot only. Clusters are separate tenancies with separate template registries, so a template built on one does not exist on the others.

template.yml splits into the DockerHub image job (unchanged, push-only) and a buildTemplate matrix job, one leg per cluster, with the cluster map in matrix.include and the key looked up by name:

matrix:
  include:
    - { cluster: foxtrot, domain: e2b.dev,         api_key_secret: E2B_API_KEY }
    - { cluster: juliett, domain: e2b-juliett.dev, api_key_secret: E2B_JULIETT_API_KEY }
    - { cluster: tango,   domain: e2b-tango.dev,   api_key_secret: E2B_TANGO_API_KEY }
env:
  E2B_API_KEY: ${{ secrets[matrix.api_key_secret] }}
  E2B_DOMAIN: ${{ matrix.domain }}

E2B_JULIETT_API_KEY and E2B_TANGO_API_KEY are new repo secrets and must be added before merging (names follow e2b-dev/code-interpreter's E2B_<CLUSTER>_API_KEY convention). A missing key fails that leg fast with Missing secret <name> for cluster: <cluster>; fail-fast: false so the other clusters still build.

Also adds a bare workflow_dispatch so the template can be rebuilt on all clusters without a template change (the Docker image only rebuilds on push). Legs on the same cluster serialize via Template-${{ github.ref }}-<cluster>; different clusters run in parallel.

Merging this PR triggers the workflow (its path filter includes the workflow file), which will do the first juliett/tango builds.

Companion PR for the code-interpreter template: e2b-dev/code-interpreter#338.

Link to Devin session: https://app.devin.ai/sessions/164235e15a6f4e30bbdc3907d388fbc5
Open in Devin Desktop: https://app.devin.ai/desktop/session/164235e15a6f4e30bbdc3907d388fbc5?variant=devin
Requested by: @mishushakov

[devin-ai-integration]

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