fix: Prevent shell injection in GitHub Actions workflow

[fix-it-felix-sentry]

Summary

This PR fixes a security finding by preventing potential shell injection in the GitHub Actions workflow.

Changes

• Replaced direct interpolation of GitHub context variables (github.sha, github.ref, github.actor, github.event_name) with environment variables
• Updated shell commands to use quoted environment variables instead of direct ${{ }} interpolation
• Applied the same fix to both the login and build steps

This follows GitHub Actions security best practices for preventing script injection attacks by treating all GitHub context data as potentially untrusted and using intermediate environment variables.

References

• Parent ticket: https://linear.app/getsentry/issue/VULN-1618
• Child ticket: https://linear.app/getsentry/issue/DI-1912
• GitHub Actions Security Hardening Guide
• Semgrep Rule

[linear-code]

DI-1912 New Semgrep Finding for getsentry/pypi

[sentry]

Bug: Assignments to reserved environment variables GITHUB_ACTOR and GITHUB_TOKEN are silently ignored, making the intended security hardening ineffective.
_{Severity: MEDIUM}

Suggested Fix

Use non-reserved names for the environment variables to ensure they are set correctly. For example, rename GITHUB_ACTOR to DOCKER_USERNAME and GITHUB_TOKEN to DOCKER_PASSWORD within the env block and update the run command to use these new variables.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: .github/workflows/image.yml#L24-L27

Potential issue: In the GitHub Actions workflow, the code attempts to set `GITHUB_ACTOR`
and `GITHUB_TOKEN` as environment variables for the `login` step. However, these are
reserved variable names in GitHub Actions, and any assignments to them are silently
ignored. Consequently, the intended security hardening—using intermediate variables to
prevent shell injection—is not applied. The `docker login` command still succeeds by
coincidence, as it falls back to using the globally available `GITHUB_ACTOR` and
`GITHUB_TOKEN` variables. This means the code does not achieve its security goal and
provides a false sense of security.

_{Did we get this right? 👍 / 👎 to inform future reviews.}