PR #193 added tiktoken preloading to Dockerfile_ecs but the same fix
was not applied to the Lambda Dockerfile. This causes a ConnectTimeout
error in network-restricted environments (e.g. Lambda in VPC without
NAT Gateway) when tiktoken tries to download cl100k_base encoding at
runtime from openaipublic.blob.core.windows.net.
Cache the encoding at build time, consistent with Dockerfile_ecs.
Related to #118
