Fix Docker build

The default nodeset was just changed to focal [1], which in the absence
of the python-is-python3 package doesn't provide a python binary.

[1] https://review.opendev.org/c/opendev/base-jobs/+/789098

Change-Id: I2b208db2b7b34da034071a4fc0c5d49768b6dad3

docker/build_image.sh

@@ -45,7 +45,7 @@ set -eo pipefail  # Exit the script if any statement returns error.
 #   $ ./build_image.sh master master refs/changes/19/595719/3
 
 # Go to folder with Docker files.
-REAL_PATH=$(python -c "import os,sys; print(os.path.realpath('$0'))")
+REAL_PATH=$(python3 -c "import os,sys; print(os.path.realpath('$0'))")
 cd "$(dirname "$REAL_PATH")/../docker/"
 
 [ -z "$DOCKER_IMAGE" ] && \