Enhance the image building doc for the DHCP based network

[mkumatag]

/kind doc
/area provider/ibmcloud

What steps did you take and what happened:
[A clear and concise description of what the bug is.]

What did you expect to happen:

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

Environment:

• Cluster-api version:
• Minikube/KIND version:
• Kubernetes version: (use kubectl version):
• OS (e.g. from /etc/os-release):

[k8s-ci-robot]

@mkumatag: The label(s) kind/doc cannot be applied, because the repository doesn't have them.

In response to this:

/kind doc
/area provider/ibmcloud

What steps did you take and what happened:
[A clear and concise description of what the bug is.]

What did you expect to happen:

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

Environment:

• Cluster-api version:
• Minikube/KIND version:
• Kubernetes version: (use kubectl version):
• OS (e.g. from /etc/os-release):

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[Amulyam24]

/assign