Use an environment variable instead of command substitution (#5364)

`$PWD` is already there so use it instead of running `pwd(1)` in a subshell each time.

Author: rjc

README.md

@@ -58,16 +58,16 @@ Download OpenStreetMap extracts for example from [Geofabrik](http://download.geo
 
 Pre-process the extract with the car profile and start a routing engine HTTP server on port 5000
 
-    docker run -t -v "$(pwd):/data" osrm/osrm-backend osrm-extract -p /opt/car.lua /data/berlin-latest.osm.pbf
+    docker run -t -v "${PWD}:/data" osrm/osrm-backend osrm-extract -p /opt/car.lua /data/berlin-latest.osm.pbf
 
-The flag `-v "$(pwd):/data"` creates the directory `/data` inside the docker container and makes the current working directory `"$(pwd)"` available there. The file `/data/berlin-latest.osm.pbf` inside the container is referring to `"$(pwd)/berlin-latest.osm.pbf"` on the host.
+The flag `-v "${PWD}:/data"` creates the directory `/data` inside the docker container and makes the current working directory `"${PWD}"` available there. The file `/data/berlin-latest.osm.pbf` inside the container is referring to `"${PWD}/berlin-latest.osm.pbf"` on the host.
 
-    docker run -t -v "$(pwd):/data" osrm/osrm-backend osrm-partition /data/berlin-latest.osrm
-    docker run -t -v "$(pwd):/data" osrm/osrm-backend osrm-customize /data/berlin-latest.osrm
+    docker run -t -v "${PWD}:/data" osrm/osrm-backend osrm-partition /data/berlin-latest.osrm
+    docker run -t -v "${PWD}:/data" osrm/osrm-backend osrm-customize /data/berlin-latest.osrm
 
 Note that `berlin-latest.osrm` has a different file extension. 
 
-    docker run -t -i -p 5000:5000 -v "$(pwd):/data" osrm/osrm-backend osrm-routed --algorithm mld /data/berlin-latest.osrm
+    docker run -t -i -p 5000:5000 -v "${PWD}:/data" osrm/osrm-backend osrm-routed --algorithm mld /data/berlin-latest.osrm
 
 Make requests against the HTTP server