Add optional 3-stage CPU temperature fan curve

[kelriclink]

Summary

This PR adds an optional 3-stage CPU temperature fan curve (simple step curve) to the Dell iDRAC fan controller container.

• Disabled by default (keeps current behavior unchanged)
• When enabled, fan speed is adjusted automatically using 3 stages based on CPU temperature

Behavior (3 stages)

The controller uses the highest detected CPU temperature (CPU1/CPU2) as T:

1. Cool (Stage 1): T < CPU_TEMPERATURE_FAN_CURVE_LOW_THRESHOLD
   → apply static fan speed CPU_TEMPERATURE_FAN_CURVE_LOW_FAN_SPEED

2. Warm (Stage 2): T > CPU_TEMPERATURE_FAN_CURVE_MID_THRESHOLD
   → apply static fan speed CPU_TEMPERATURE_FAN_CURVE_MID_FAN_SPEED

3. Hot (Stage 3 / Safety): T > CPU_TEMPERATURE_THRESHOLD
   → switch back to Dell default dynamic fan control profile (auto)

Between LOW and MID thresholds, the last applied curve stage is kept to reduce oscillation.

New environment variables

• ENABLE_CPU_TEMPERATURE_FAN_CURVE (default: false)
• CPU_TEMPERATURE_FAN_CURVE_LOW_THRESHOLD (default: 60)
• CPU_TEMPERATURE_FAN_CURVE_LOW_FAN_SPEED (default: 12)
• CPU_TEMPERATURE_FAN_CURVE_MID_THRESHOLD (default: 75)
• CPU_TEMPERATURE_FAN_CURVE_MID_FAN_SPEED (default: 20)

Fan speeds support decimal (0–100%) or hex (0x00–0x64), same as FAN_SPEED.

Note: when the curve is enabled, FAN_SPEED is ignored.

Implementation notes

• Reuses existing IPMI raw commands for switching auto/manual and setting manual fan percentage.
• Fixes an issue when CPU2 sensor is absent (avoid printing/format errors).
• Documentation updated with usage examples.

Files changed

• Dell_iDRAC_fan_controller.sh: adds curve logic + selection
• functions.sh: adds apply_user_fan_control_profile_with_speed helper + small sensor output fix
• Dockerfile: adds default ENV values for the new variables
• README.md: documents the feature and parameters

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l have already test it at my dell r730xd ,it works great.