dnscrypt-proxy Docker image

This is a no-frills Docker image for dnscrypt-proxy. The configuration is lightly modified from the supplied example-dnscrypt-proxy.toml:

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< listen_addresses = ['127.0.0.1:53']
---
> listen_addresses = ['0.0.0.0:53']
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< # user_name = 'nobody'
---
> user_name = 'nobody'
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< ipv6_servers = false
---
> ipv6_servers = true