static-exporter: Update httpd.conf file to include headers

Prometheus v3.0.0 has a stricter requirements on the headers that are
returned from exporters. The static-exporter does not set any headers
and causes scrapes to fail with the following error:

```
non-compliant scrape target sending blank Content-Type and no fallback_scrape_protocol specified for target ````
```

This adds an httpd.conf file to mount to the container that includes a
directive to add a header that solves this problem. To generate the
httpd.conf file, I ran the following command:

`docker run --rm httpd:2.4 cat /usr/local/apache2/conf/httpd.conf > my-httpd.conf`

Author: Pokom