EdgeX DevOps: General Dockerfile Standardization

[ernestojeda]

During Ireland planning DevOps wanted to align some aspects of the Dockerfiles across all repositories to have more uniformity between Dockerfiles. It was agreed upon that the following changes would take place in the Dockerfiles.

1. For build stage, we will use Go 1.15 with 3.12 (golang:1.15-alpine3.12) Alpine where applicable (for Go services)
2. Move to alpine 3.12 (in second stage FROM in Dockerfile)
3. Align to similar apk add commands. i.e. apk add --update --no-cache ... vs. apk update && apk add ... && rm -rf /var/cache/apk/*