oci: Use Enum Mode.DOCKER instead of str comparision

Dom Rodriguez (Codethink)requested to merge
domrodriguez/fix-enum-mode-oci into master

The oci plugin was comparing the mode variable to a string of "docker" instead of the Enum itself.

When using this with a element, I noticed that this method was causing issues in comparison, and this change fixed it.

This MR is a draft as a temporary fix, whilst I investigate further.

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