Make Docker image easier to use (#50)

* Use single server example as default frontend

This just works out of the box

* Refactor Dockerfile

- Remove the asset dir since it's embedded in the executable anyway
- Use the code in the current directory to build instead of downloading.
  Makes local testing very easy.
- Cache Go dependencies
- Run without root privileges. Note that using a BoltDB database
  requires mounting a volume with the correct permissions. The default
  database has thus been changed to memory so that starting the image
  just works.

* Automatically publish images

settings.toml

@@ -22,7 +22,7 @@ redact_ip_addresses=false
 
 # database type for statistics data, currently supports: none, memory, bolt, mysql, postgresql
 # if none is specified, no telemetry/stats will be recorded, and no result PNG will be generated
-database_type="bolt"
+database_type="memory"
 database_hostname=""
 database_name=""
 database_username=""
@@ -37,4 +37,4 @@ enable_http2=false
 
 # if you use HTTP/2 or TLS, you need to prepare certificates and private keys
 # tls_cert_file="cert.pem"
-# tls_key_file="privkey.pem"
+# tls_key_file="privkey.pem"