Add additional option to skip ssh-keyscan, update README

Author: michael.biriley@firstaml.com

README.md

@@ -20,6 +20,10 @@ Paths accepted by `git sparse-checkout set`.
 
 Whether to pass `--no-cone` to `git sparse-checkout` so that the paths are considered to be a list of patterns.
 
+#### `skip_ssh_keyscan` ('true' or 'false')
+
+Whether to skip ssh-keyscan step. This will skip adding each ssh public key into the known-hosts file. Only use if ssh keys are already setup.
+
 ## Example
 
 Below is an example for using sparse-checkout plugin.

hooks/checkout

@@ -11,16 +11,20 @@ NO_CONE_OPTION="$(plugin_read_config NO_CONE "false")"
 NO_CONE_PARAM=""
 [[ $NO_CONE_OPTION = false ]] || NO_CONE_PARAM="--no-cone"
 
+SKIP_SSH_KEYSCAN_OPTION="$(plugin_read_config SKIP_SSH_KEYSCAN "false")"
+
 if plugin_read_list_into_result PATHS; then
     CHECKOUT_PATHS=("${result[@]}")
 else
     echo "Missing 'paths' option in the plugin"
     exit 1
 fi
 
-echo "Scanning SSH keys for remote git repository"
-[[ -d ~/.ssh ]] || mkdir -p ~/.ssh
-ssh-keyscan "${BUILDKITE_REPO_SSH_HOST}" >> ~/.ssh/known_hosts
+if [[ "$SKIP_SSH_KEYSCAN_OPTION" != "false" ]]; then
+    echo "Scanning SSH keys for remote git repository"
+    [[ -d ~/.ssh ]] || mkdir -p ~/.ssh
+    ssh-keyscan "${BUILDKITE_REPO_SSH_HOST}" >> ~/.ssh/known_hosts
+fi
 
 echo "Creating sparse-checkout with paths: ${CHECKOUT_PATHS[*]}"