container: add --health-cmd-mode for CMD healthcheck form

[lohitkolluri]

docker run/create always wraps --health-cmd in CMD-SHELL, which
fails on scratch and other shell-less images (#3719).

Add --health-cmd-mode with two values:

Value	Behavior
shell (default)	wraps command in CMD-SHELL — identical to existing behavior
exec	uses the exec form (CMD), required for images without a shell

This mirrors the CMD / CMD-SHELL distinction in Dockerfile HEALTHCHECK instructions.

Usage

# exec form for scratch / distroless
docker run --health-cmd=/healthcheck --health-cmd-mode=exec my-image

# multi-argument exec form (parsed with shlex)
docker run --health-cmd='/usr/bin/wget -q -O /dev/null http://localhost/' \
           --health-cmd-mode=exec my-image

# explicit shell form — same as omitting --health-cmd-mode
docker run --health-cmd='curl -f http://localhost/' --health-cmd-mode=shell my-image

Test plan

• go test ./cli/command/container/... -run TestParseHealth
• make mddocs (docs regenerated in this commit)

Fixes #3719

[andrebrait]

How about an option --health-cmd-mode with values shell (the default) and exec?

Just in case the future has even more modes possible, so changing this is easier?

[andrebrait]

Or cmd instead of exec, or upper-case, etc.

Up to the maintainers to know what they prefer, but I'm just throwing the idea out there.

[lohitkolluri]

@andrebrait Great suggestion — updated to --health-cmd-mode with string values shell (default, keeps CMD-SHELL behavior) and exec (uses the CMD form). This is cleaner and leaves room for future modes.

Changes in the latest commit:

• --health-cmd-exec bool → --health-cmd-mode string (default "shell")
• Error on unrecognised mode value
• Tests updated for both modes + invalid-mode case
• Docs regenerated via make mddocs

[codecov-commenter]

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