Update BASE_IMAGE in Dockerfile, closes #799

docker/base/Dockerfile

@@ -1,5 +1,5 @@
 # Set base image
-ARG BASE_IMAGE=ubuntu:20.04
+ARG BASE_IMAGE=python:3.9-slim
 FROM $BASE_IMAGE
 
 # Install GPU-enabled version of PyTorch if set
@@ -9,7 +9,7 @@ ARG GPU
 ARG TARGETARCH
 
 # Set Python version (i.e. 3, 3.9)
-ARG PYTHON_VERSION=3.9
+ARG PYTHON_VERSION=3
 
 # List of txtai components to install
 ARG COMPONENTS=[all]