Update Node from 12.4.0 to 12.18.4 in the image for gitlab-org/customers-gitlab-com

Updates Node.js from 12.4.0 to 12.18.4 in the image for gitlab-org/customers-gitlab-com.

Part of #62 (closed)

Relates to !374 (merged)

Blocks gitlab-org/customers-gitlab-com!2874

Signed-off-by: Takuya Noguchi takninnovationresearch@gmail.com