fix: update go package builds to satisfy golang 1.16

[ynohat]

Go 1.16 enables modules by default. We now git clone packages and

• if a go.mod file is not provided by that module, we run go mod init
• depending on the module, we run go mod tidy or go mod vendor

Ideally, we'd detect the case appropriately but unsure how to do this at this point.

Additionally, this PR includes a change to docker login before building these images in an attempt to lift our docker pull quota restrictions.

Merged into dev already, here is the travis build: https://travis-ci.com/github/akamai/akamai-docker/builds/217525501

@javiergarza @akamaimike fyi, but not requiring your review for this PR