Rewritten Dockerfile into a better image

[ricardbejarano]

I've rewritten the exporter's Dockerfile into a better image:

• Runs as a non-root user, which adds a layer of protection should the container runtime have a vulnerability that requires root privileges inside the container (which has happened before)
• Builds ssl_exporter during build time, so that Docker Hub builds can be trusted, instead of trusting the maintainers to docker push authentic binaries
• Does not include busybox and all it's tools, which go unused inside the image but could be leveraged by an attacker should the image be flawed
• A bit more efficient in space (about 1.2MB less)
• Adds an EXPOSE clause for 9219/tcp, just for correctness
• The image's filesystem tree is less tall (as you can see using dive):

/
├── etc/
│   ├── group
│   ├── passwd
│   └── ssl/
│       └── certs/
│           └── ca-certificates.crt
└── ssl_exporter

[ricardbejarano]

I could host the image in the Docker Hub for anyone to test before merging, if need be. I've tested it locally and it works as expected.

[ribbybibby]

Looks good to me @ricardbejarano. Thanks for the contribution!