Update default Ubuntu image

Author: ocaisa

easybuild/tools/config.py

@@ -134,7 +134,7 @@
 JOB_DEPS_TYPE_ABORT_ON_ERROR = 'abort_on_error'
 JOB_DEPS_TYPE_ALWAYS_RUN = 'always_run'
 
-DOCKER_BASE_IMAGE_UBUNTU = 'ubuntu:16.04'
+DOCKER_BASE_IMAGE_UBUNTU = 'ubuntu:20.04'
 DOCKER_BASE_IMAGE_CENTOS = 'centos:7'
 
 LOCAL_VAR_NAMING_CHECK_ERROR = 'error'