Add Dockerfile

[mojca]

We used @Korusuke's work as the basis to create a slightly more improved Docker image for MacPorts base and suggest to merge the image to the base where it makes most sense.

What's left to be done is:

• fix the configure arguments
• reduce the number of runtime dependencies to just the required ones (most of dependencies are probably just build dependencies, but this needs to be checked)
• fix Portfiles which fail at portindex