Make systemd unit configurable and fix :system-mode defaults

[floriancrusius]

Summary

• Configurable env vars in the systemd unit — adds :solid_queue_service_unit_env_files and :solid_queue_service_unit_env_vars (inspired by capistrano-sidekiq). Both fall back to shared :service_unit_env_files / :service_unit_env_vars so multi-plugin setups can share one source.
• Configurable command — :solid_queue_command (default "rake solid_queue:start") so SolidQueue >= 0.4 setups can switch to "bin/jobs" without templating.
• Fix [Install] WantedBy= — now multi-user.target in :system mode (previously always default.target, which prevented boot activation of system units).
• Fix :system mode running as root — now renders User=<solid_queue_user> (defaults to :run_as, then Capistrano's :user).
• Breaking: Capistrano's default_env is no longer auto-rendered into the unit; MALLOC_ARENA_MAX=2 is no longer hardcoded. Pass what you need explicitly via the new hooks. RAILS_ENV is still derived from the current stage.

Test plan

• bundle exec cap <stage> solid_queue:generate and inspect the generated solid_queue.service — verify Environment=, EnvironmentFile=, ExecStart=, User=, WantedBy= look correct in both :user and :system modes.
• bundle exec cap <stage> solid_queue:install on a staging host, systemctl --user status <unit> (or sudo systemctl status … for :system) — service comes up.
• Set :solid_queue_service_unit_env_vars, ["FOO=bar"] and confirm it lands in the rendered unit.
• Set :solid_queue_command, "bin/jobs" on a SolidQueue-0.4+ app and confirm the process starts.

[Copilot]

Pull request overview

This PR enhances the SolidQueue systemd integration by making the generated unit file more configurable (command + environment injection) and by correcting :system-mode unit defaults so services can be enabled at boot and run as a non-root user.

Changes:

• Add configuration for the systemd unit’s environment (:solid_queue_service_unit_env_files / :solid_queue_service_unit_env_vars) with fallbacks to shared :service_unit_env_* variables.
• Add :solid_queue_command to control what runs after bundle exec (defaulting to rake solid_queue:start).
• Adjust :system mode rendering (set User= and use WantedBy=multi-user.target) and document the breaking removal of rendering Capistrano default_env into the unit.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
README.md	Documents new configuration knobs for command, user, and unit environment injection.
lib/capistrano/templates/solid_queue.service.erb	Updates the unit template to use configurable env/files, configurable command, and corrected :system defaults (User=, WantedBy=).
lib/capistrano/solid_queue.rb	Introduces new default variables for command/user/unit env sources.
CHANGELOG.md	Records new configuration features, fixes, and breaking behavior change.

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