Add a docker container & run tests there

This is a bit of a weird idea:  We run tests as part of the docker build.

.travis.yml

@@ -5,19 +5,14 @@ language: go
 go:
   - '1.12.x'
 
-install:
-  - pip install cram --user
-
-before_script:
-  - go vet -v ./...
+services:
+ - docker
 
 script:
-  - GOOS=linux GOARCH=amd64 go build -o certigo-linux-amd64
-  - GOOS=darwin GOARCH=amd64 go build -o certigo-darwin-amd64
-  - GOOS=windows GOARCH=amd64 go build -o certigo-windows-amd64.exe
-  - go test -v ./...
-  - ln -s certigo-linux-amd64 certigo
-  - PATH=$PWD:$PATH cram -v tests/*.t
+  - docker build -f Dockerfile .
+  - GOOS=linux GOARCH=amd64 go build -o certigo-$GOOS-$GOARCH
+  - GOOS=darwin GOARCH=amd64 go build -o certigo-$GOOS-$GOARCH
+  - GOOS=windows GOARCH=amd64 go build -o certigo-$GOOS-$GOARCH
 
 deploy:
   provider: releases

Dockerfile

@@ -0,0 +1,21 @@
+FROM golang:alpine AS builder
+
+COPY . /src
+WORKDIR /src
+ENV CGO_ENABLED=0
+RUN go build -mod=vendor -o /bin/certigo
+RUN go vet -mod=vendor ./...
+RUN go test -mod=vendor ./...
+
+FROM python
+RUN pip install cram
+
+COPY --from=builder /bin/certigo /bin/certigo
+
+COPY tests /tests
+RUN cram -v /tests/*.t
+
+FROM alpine
+RUN apk update && apk add ca-certificates && rm -rf /var/cache/apk/*
+COPY --from=builder /bin/certigo /bin/certigo
+ENTRYPOINT ["/bin/certigo"]