It appears that the `go build` command was pointing to the wrong directory - the build was failing as follows:
```
...
 => ERROR [builder 7/8] RUN go build -ldflags="-w -s" -o /go/bin/certstream-server-go /go/src/certstream-server-go/cmd
------
 > [builder 7/8] RUN go build -ldflags="-w -s" -o /go/bin/certstream-server-go /go/src/certstream-server-go/cmd:
no Go files in /go/src/certstream-server-go/cmd
```

After it was fixed, the configfile had 0750 permissions - it was owned by root:root, thus certstream-server-go was failing to start.
```
config.go:49: Error while parsing yaml file: open /app/config.yaml: permission denied
```
This commit resolves this issue too.

Ensure that Dockerfile_multistage builds and is runnable

Before this change the websocket server would not have any heartbeat mechanism,
 this meant that unless the client proactively implements heartbeats the server
 would close the connection. From what I have seen most non-browser clients do
 not implement heartbeats (e.g filebeat, websocat), thus for those clients the
 connection would have been closed in 65 seconds.

The Websocket RFC does not set out a requirement which side should initiate
 heartbeats or that they are required. Most browsers have implemented heartbeats
 from client side, however it is not a must thus, in my opinion it is beneficial
 for the server to implement them, especially if the server closes connection if
 heartbeat is not received. As a consequence this changeset allows to support
 clients which aren't browsers.

This commit adds the ability to enable compression for websockets.
The compression is disabled by default due to the fact that it is marked
as experimental in gorilla/websocket and there have been issues with
compression whenever Safari is enabled.
This commit partially addresses #30.

While this is great for debugging purposes, this rather spams the log with a large number of clients.

I ignored them before, and even though it's unlikely for errors to occur here, It's better to handle them properly.

Add support for server heartbeats

I appreciate the comment as explanation, but using regular bools in the struct will lead to the default values being parsed. The default value for bools is "false". So there's no need to use a pointer in here.

Add support for Websocket compression

fixes #42

We also needed a way to properly prevent the entries of the metrics map from being reset to 0. That's why I added the Init() method to the LogMetrics. It only sets the value of the operator/url combination to 0 if it doesn't exist before.

If it already exists, it will be left alone.