Docker: exec single commands

This replaces bash with the docker command, freeing memory and
automatically returning the exit code of the docker command as the exit
code of the shell script.

docker/1_base/build.sh

@@ -1,3 +1,3 @@
 #!/usr/bin/env bash
 
-docker build -t pillar_base -f base.docker .;
+exec docker build -t pillar_base -f base.docker .;