Docker builds need to use dep to manage dependencies

Dockerfile

@@ -1,11 +1,19 @@
 FROM golang:1.11-alpine3.8 AS downloader
 ARG VERSION
 
-RUN apk add --no-cache git gcc musl-dev
+RUN apk add --no-cache git gcc musl-dev curl
+
+RUN curl -fsSL -o /usr/local/bin/dep https://github.com/golang/dep/releases/download/v0.4.1/dep-linux-amd64 && chmod +x /usr/local/bin/dep
 
 WORKDIR /go/src/github.com/golang-migrate/migrate
 
-COPY . ./
+COPY Gopkg.toml Gopkg.lock ./
+RUN dep ensure -vendor-only
+
+COPY *.go ./
+COPY cli ./cli
+COPY database ./database
+COPY source ./source
 
 ENV DATABASES="postgres mysql redshift cassandra spanner cockroachdb clickhouse"
 ENV SOURCES="file go_bindata github aws_s3 google_cloud_storage"