Add checks for empty continuation lines, healthcheck syntax & multiple declarations of healthcheck and stopsignal.

[svl7]

• Detect empty continuation lines in multi-line RUN commands. It's
  currently deprecated and will lead to errors in future releases of
  Docker, see:
  moby/moby@2fd736a

• Check syntax of HEALTHCHECK command according to Docker documentation

• Detect multiple declarations of HEALTHCHECK and STOPSIGNAL

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