Ensure that the configuration file is an absolute path in Docker build

The specific problem is in commit 2ddd7c1, where the passed config file
(using the `-c` option) is now mounted inside the container using the
`--volume src:dest:opt` Docker option.

The problem is that Docker requires absolute paths for mounting single
files inside the container, otherwise it silently tries to mount a volume
name instead as an empty directory. Therefore the Docker build no longer
works with the following invocation forms (relative config-paths):

    ./build-docker.sh -c myconfig
    /path/to/build-docker.sh -c myconfig   # also doesn't work

This commit uses `realpath` (included in coreutils) in the Docker build
script to ensure that the passed configuration file is always an
absolute path before passing it to Docker.

build-docker.sh

@@ -30,6 +30,9 @@ do
 	esac
 done
 
+# Ensure that the configuration file is an absolute path
+CONFIG_FILE=$(realpath -s "$CONFIG_FILE")
+
 # Ensure that the confguration file is present
 if test -z "${CONFIG_FILE}"; then
 	echo "Configuration file need to be present in '${DIR}/config' or path passed as parameter"