Override entrypoint in docker_service resource

[mavogel]

This issue was originally opened by @b4nst as hashicorp/terraform-provider-docker#261. It was migrated here as a result of the community provider takeover from @kreuzwerker. The original body of the issue is below.

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Terraform Version

Terraform v0.12.24

• provider.docker v2.7.0

Affected Resource(s)

• docker_service

Expected Behavior

Resource accept entrypoint option, as specified in docker service_create doc

Actual Behavior

Resource does not accept an entrypoint option

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[github-actions]

This issue is stale because it has been open 60 days with no activity.
Remove stale label or comment or this will be closed in 7 days.
If you don't want this issue to be closed, please set the label pinned.

[mavogel]

AFAICS in the cli v20.10.7 the entrypoint flag of the cli is passed to the command property of the ServiceSpec -> TaskSpec -> ContainerSpec, see here https://github.com/docker/cli/blob/v20.10.7/cli/command/service/opts.go#L705

We actually do support this already by the command attribute. We could

1. add an entrypoint attribute, let it conflict with the command. Then deprecate and remove the command in v3.0.0.
2. or update the documentation only

@suzuki-shunsuke WDYT?