Add zip to openwhisk/swift3action Docker image

[akrabat]

The most reliable way to create a Swift binary to run on OpenWhisk is to use the same Docker container that OpenWhisk itself uses (openwhisk/swift3action).

The steps to compile are documented at Packaging an action as a Swift executable and the first step is to install zip:

apt-get install -y zip

This takes a little time as it has to pull it from the network, and you have to do this every time you compile every action.

It would therefore be convenient if openwhisk/swift3action had zip installed by default.