DL3012 : Multiple HEALTHCHECK instructions

Description

Each build stage may define at most one HEALTHCHECK. Additional HEALTHCHECK instructions override previous ones and can be confusing.

Goals

• Keep Dockerfiles clear about active health checks.
• Prevent accidental masking of earlier HEALTHCHECK instructions.

Specification

1. Traverse instructions in order.
2. Reset a stage-scoped flag whenever a FROM instruction appears.
3. Upon encountering a HEALTHCHECK:
4. If a HEALTHCHECK has already been seen in the current stage, emit DL3012 at that line.
5. Otherwise, mark that a HEALTHCHECK has been seen for this stage.

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