Tighten Docker package regexp

Previously 19.0.3.1 would match multiple versions:

```
5:19.03.11~3-0~debian-stretch
5:19.03.10~3-0~debian-stretch
5:19.03.1~3-0~debian-stretch
```

We can tighten up the regexp by adding a `~` to the end of
the version.

scripts/install-docker

@@ -19,7 +19,7 @@ curl -fsSL https://download.docker.com/linux/debian/gpg | apt-key add -
 echo "deb [arch=amd64] https://download.docker.com/linux/debian stretch stable" >> /etc/apt/sources.list.d/docker.list
 apt-get update
 
-PACKAGE_VERSION=$(apt-cache policy docker-ce | awk -v dv=${DOCKER_VERSION} '$1 ~ dv {print $1}')
+PACKAGE_VERSION=$(apt-cache policy docker-ce | awk -v dv=${DOCKER_VERSION}~ '$1 ~ dv {print $1}')
 
 apt-get install -y docker-ce=${PACKAGE_VERSION} docker-ce-cli=${PACKAGE_VERSION}
 apt-get -yq autoremove