Add VC_ENABLE_RDS_OFFHOST build-arg for Alpine Linux

[robbwagoner]

Per the alpine/README.md, this adds an optional Docker build-arg VC_ENABLE_RDS_OFFHOST to configure the VividCortex agent to enable query digest and sampling via the PERFORMANCE_SCHEMA. It is intended for use with AWS RDS MySQL and RDS Aurora MySQL since the setting is not exposed via RDS Parameter Groups, per the VividCortex documentation.

Usage: --build-arg VC_ENABLE_RDS_OFFHOST=1.
Creates /etc/vividcortex/vc-mysql-metrics.conf:

  {
    "force-offhost-digests": "true",
    "force-offhost-samples": "true"
  }