deps-conda

[bertsky]

This starts Conda with deps-conda as a replacement for Apt with deps-ubuntu to install system dependencies.

System dependencies should be encapsulated better than via fixed Linux distributions in OCR-D. Long ago I expressed my conviction that we can find some existing universal mechanism for porting. IMHO Conda fits that description quite well.

So the idea is to allow OCR-D modules to express their system requirements in a deps-conda rule, which will run conda install .... Of course, modules could do even more and provide a full conda build (i.e. build.sh and meta.yaml) and have the Makefile simply delegate to that.

For example, the ocrd/tesserocr Dockerfile could be as simple as

FROM ocrd/core
COPY Makefile .
RUN make deps-conda
RUN pip install .

where that would simply be defined as

deps-conda:
	conda install -c conda-forge tesseract leptonica

(which would give us an up to date libtesseract against which pip install tesserocr will compile).

To make that work, this PR lays the foundation in ocrd/core:

• install Conda, if not already present
• install system dependencies for ocrd, ocrd_utils etc. via its own deps-conda rule

Techically, we could separate the second commit (i.e. switching from apt to conda in the Dockerfiles) and save it for another day (when we have the courage).