Post the Docker image build results to the shared CI Slack channel

[albertvillanova]

This PR sends the Docker image build results to the shared CI Slack channel, like every other workflow.

Motivation

docker-build.yml was the only workflow pointing to its own channel, as noted in revision #6895 (comment). Every other workflow declares CI_SLACK_CHANNEL once at the workflow level and passes it at each call site, so the thirteen call sites did not all resolve the channel the same way.

Note that this changes where those notifications are delivered.

To do on merge

• Delete the now unused CI_DOCKER_CHANNEL repository secret. It is no longer referenced anywhere once this PR is merged, and deleting it is a repository setting rather than something this PR can do.

Changes

• Declare CI_SLACK_CHANNEL at the workflow level in docker-build.yml, from the same secret as the other workflows
• Use it in both Post to Slack steps, in place of the Docker specific channel

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Note

Low Risk
Only changes which Slack channel receives CI notifications; no runtime or build logic changes.

Overview
Aligns Docker image build Slack notifications with the rest of the repo’s CI workflows.

docker-build.yml now sets workflow-level CI_SLACK_CHANNEL from CI_PUSH_MAIN_CHANNEL (same pattern as workflows like slow-tests.yml). Both Post to Slack steps (trl and trl-dev) pass env.CI_SLACK_CHANNEL instead of CI_DOCKER_CHANNEL, so build results land in the shared CI channel rather than the Docker-only channel. After merge, CI_DOCKER_CHANNEL can be removed from repo secrets.

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[bot-ci-comment]

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[albertvillanova]

Merging this PR as this just addresses a specific request by @qgallouedec: #6895 (comment)

it would be easier if everything were in the same channel: this is the only action pointing to its own channel: secrets.CI_DOCKER_CHANNEL