Set version automatically

Convert the check script to a script that sets the version to match
whatever Riot is being packaged.

Add a README because the two ways of interacting with versions are
starting to get complex to reason about.

Also add a docker build script.

scripts/dockerbuild.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+
+# Taken from https://www.electron.build/multi-platform-build#docker
+docker run --rm -ti \
+ --env-file <(env | grep -iE 'DEBUG|NODE_|ELECTRON_|YARN_|NPM_|CI|CIRCLE|TRAVIS_TAG|TRAVIS|TRAVIS_REPO_|TRAVIS_BUILD_|TRAVIS_BRANCH|TRAVIS_PULL_REQUEST_|APPVEYOR_|CSC_|GH_|GITHUB_|BT_|AWS_|STRIP|BUILD_') \
+ --env ELECTRON_CACHE="/root/.cache/electron" \
+ --env ELECTRON_BUILDER_CACHE="/root/.cache/electron-builder" \
+ -v ${PWD}:/project \
+ -v ${PWD##*/}-node-modules:/project/node_modules \
+ -v ~/.cache/electron:/root/.cache/electron \
+ -v ~/.cache/electron-builder:/root/.cache/electron-builder \
+ electronuserland/builder yarn run build