Add Dockerfile and Makefile

[dalf]

make build to build a docker image filtron:latest (no need to install golang, it uses a muti-stage Dockerfile)

make run to start filtron, help to debug.

should help searx/searx#1561

the configuration file is from https://www.21x9.org/en/searx.html

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Note that there are still things to do :

• run filtron with a dedicated user instead of root
• load the configuration from a volume : when there is no configuration, a default configuration is copied. It allow the administrator to start quickly, and customize later.
• compile different docker images automatically on hub.docker.com for amd64, x64, arm, arm64 architectures.

[asciimoo]

Thanks!