Dockerfile updates after open sourcing the repo

We no longer need SSH_PRIVATE_KEY argument while generating docker
images.

Dockerfile

@@ -26,20 +26,7 @@ RUN go install
 # Build Cadence binaries
 FROM golang:1.13.6-alpine AS builder
 
-RUN apk add --update --no-cache ca-certificates make curl git mercurial bzr openssh protobuf
-
-# add credentials to builder
-ARG SSH_PRIVATE_KEY
-RUN mkdir /root/.ssh/
-RUN echo "${SSH_PRIVATE_KEY}" > /root/.ssh/id_rsa
-
-RUN chmod 600 /root/.ssh/id_rsa
-
-# make sure domain is accepted
-RUN touch /root/.ssh/known_hosts
-RUN ssh-keyscan github.com >> /root/.ssh/known_hosts
-
-RUN git config --global url."[email protected]:".insteadOf "https://github.com/"
+RUN apk add --update --no-cache ca-certificates make curl git mercurial bzr protobuf
 
 # Making sure that dependency is not touched
 ENV GOFLAGS="-mod=readonly"