chore: some improvements to build automation

Dockerfile

@@ -7,9 +7,13 @@ RUN apk add --no-cache git
 
 RUN mkdir -p /tmp/drone-terraform
 WORKDIR /tmp/drone-terraform
-COPY . .
 
+COPY go.mod go.mod
+COPY go.sum go.sum
 RUN go mod download
+
+COPY . .
+
 RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -a -tags netgo -o /go/bin/drone-terraform
 
 FROM alpine:3.9

build-docker.sh

@@ -1,5 +1,7 @@
 #!/bin/bash
 
+set -e
+
 tag=$1
 
 if [ -z $tag ]; then
@@ -25,9 +27,9 @@ if [[ "$ans" != "Y" && "$ans" != "y" ]]; then
   exit 0
 fi
 
+set -x
 docker build -t jmccann/drone-terraform:latest .
 
-set -x
 docker tag jmccann/drone-terraform:latest jmccann/drone-terraform:${major}
 docker tag jmccann/drone-terraform:latest jmccann/drone-terraform:${major}.${minor}
 docker tag jmccann/drone-terraform:latest jmccann/drone-terraform:${major}.${minor}-${tf_ver}