Explicit solc install required for client docker image

[dtebbs]

Recent versions of the solc binary do not run alpine linux, which causes the client setup to fail.
The compiler install process (client/zeth/core/contracts.py install_solc) has been updated to fall back to installing from source (which works, but takes a long time), and Dockerfile-client extracts an alpin-compatible binary from an ethereum/solc docker image (placing it in the appropriate place, so that install_solc becomes a no-op.

This requires the compiler version to be replicated in Dockerfile-client, and this must be maintained to match the value in client/zeth/core/constants.py, otherwise the alpine binary will be unused and install_solc will perform a source install (instead of throwing an error).

If the solc binary releases support alpine in the future we can remove the explicit install in Dockerfile-client. Alternatively, we could use a script of some kind to retrieve the alpine binary based on the version in constants.py (note the this script probably has to run outside of the container if it extracts from the docker image).