build.sh: accept an optional graphhopper tag

```
Build a docker image

Usage:
  ./build.sh [<tag> | --help]

Argument:
  <tag>   Build an image for the given graphhopper repository tag [default: master]

Option:
  --help  Print this message
```

build.sh

@@ -1,6 +1,39 @@
 #!/bin/bash
 
-echo "Cloning graphhopper"
-git clone https://github.com/graphhopper/graphhopper.git
-echo "Building docker image"
-docker build . -t israelhikingmap/graphhopper:latest
+usage() (
+cat <<USAGE
+Build a docker image
+
+Usage:
+  ./build.sh [<tag> | --help]
+
+Argument:
+  <tag>   Build an image for the given graphhopper repository tag [default: master]
+
+Option:
+  --help  Print this message
+USAGE
+)
+
+if [ $# -gt 1 ] || [ "$1" == "--help" ]; then
+  usage
+  exit
+fi
+
+if [ ! -d graphhopper ]; then
+  echo "Cloning graphhopper"
+  git clone https://github.com/graphhopper/graphhopper.git
+fi
+
+imagename="israelhikingmap/graphhopper:${1:-latest}"
+if [ "$1" ]; then
+  echo "Checking out graphhopper:$1"
+  (cd graphhopper; git checkout --detach "$1")
+fi
+
+echo "Building docker image ${imagename}"
+docker build . -t ${imagename}
+
+if [ $# -eq 1 ]; then
+  echo "Use \"docker push ${imagename}\" to publish the image on Docker Hub"
+fi