Pin versions of alpine and node-modules

[CogsAndGears]

More recent versions of golang:alpine fail to compile due to go get being deprecated in favour of go install, so this pins the golang container to 1.15, when go get was still supported. Also, it copies over the package-lock.json file from the webapp folder as part of the docker build so that the npm modules also get their versions pinned to ones that still work; otherwise there were some dependency tree issues while trying to install node modules that would cause the docker build to fail.

I tried going up to golang:1.16-alpine, but npm build in 1.16 was arguing with me and I didn't have time to explore it further, so left the image pinned to golang:1.15-alpine, which seems to work out of the box.