Dockerfile: improve apt-get

Dockerfile

@@ -1,14 +1,16 @@
 FROM golang:1.10.3-stretch AS build
 WORKDIR src/github.com/mesg-foundation/core
 COPY . .
-
 ARG version
 RUN go build -o mesg-core \
       -ldflags="-X 'github.com/mesg-foundation/core/version.Version=$version'" \
       core/main.go
 
 FROM ubuntu:18.04
-RUN apt-get update && apt-get install -y ca-certificates
+RUN apt-get update && \
+      apt-get install -y --no-install-recommends ca-certificates=20180409 && \
+      apt-get clean && \
+      rm -rf /var/lib/apt/lists/*
 WORKDIR /app
 COPY --from=build /go/src/github.com/mesg-foundation/core/mesg-core .
 CMD ["./mesg-core"]