Add Dockerfile

[lemmarathon]

Addresses issue #508

To build the Docker image (in repo root):

docker build -t galoisinc/cryptol .

To pass build arguments to make, use --build-args:

docker build --build-args TESTS= --build-args DIFF=meld -t galoisinc/cryptol .

To pull the current development image:

docker pull lemmarathon/cryptol

To run cryptol:

docker run -it lemmarathon/cryptol

Tasks:

• Pass all tests
• Build as unprivileged user
• Pass options (e.g. DIFF, TESTS) to make via build-args
• Remove unnecessary layers
• Base runtime image on scratch image? Current (alpine-based) image is 38 24 17 MB compressed, 74 54 MB uncompressed. Uncompressed size could be reduced to 40 MB (see output below), but it's probably not worth the extra build steps.

# du -csh $(echo $(ldd `which cryptol`) $(ldd `which z3`) | grep -o '/[^ ]*' | sort | uniq) /usr/local /home/cryptol /usr/bin/z3
552.0K  /lib/ld-musl-x86_64.so.1
4.0K    /usr/lib/libffi.so.6
72.0K   /usr/lib/libgcc_s.so.1
4.0K    /usr/lib/libgmp.so.10
4.0K    /usr/lib/libgomp.so.1
4.0K    /usr/lib/libncursesw.so.6
4.0K    /usr/lib/libstdc++.so.6
18.2M   /usr/local
28.0K   /home/cryptol
20.6M   /usr/bin/z3
39.5M   total

[lemmarathon]

Currently Works on My Machine™. Additional testing needed (e.g. sharing data with host via volumes). Size is 27 MB compressed, 74 MB uncompressed. libz3.so is 20.5 MB, however, and doesn't seem to be required, according to ldd. Retesting without libz3.so to see if we can save some extra MB.

[lemmarathon]

Squashed commits and rebased on current master. This PR is ready for review by a maintainer. @atomb?