docker: simplify apk add lines (#759)
The images produced are no longer consumed directly by sourcegraph so we
don't have security scanners running on them. Additionally we upgraded
to the latest alpine now. Both combined means we can simplify the apk
add lines.
Test Plan: built locally
docker build -t zoekt .
docker build -t zoekt-indexserver . -f Dockerfile.indexserver
docker build -t zoekt-webserver . -f Dockerfile.webserver