Speed up docker build process

[ChihChengLiang]

• need a new flow of pushing hot fix to dockerhub.
• try not to go get packages everytime.

[ChihChengLiang]

Here's an idea to save the painfully go get time. Run go get for the online repo first and run again after copying codebase. The second go get is fast since it only gets the newly introduced packages if any.

FROM golang:1.10.3-alpine
# ... apk add and install gx, etc. 

# Go get here first
RUN go get -v github.com/ethresearch/sharding-p2p-poc

# ...

COPY *.go ./
COPY pb ./pb
# After copy everything, run go get again.
RUN go get -d -v .

# ...

[ChihChengLiang]

wow, there's a go mod feature introduced in Go 1.11, which allows us to export a go.mod and a go.sum file. So in the Dockerfile, the source code change would not ruin the cache of dependencies.

# ...
COPY go.mod .
COPY go.sum .

RUN go mod download
COPY . .

example