Based docker build on alpine:edge image and included tests for the build

in the Dockerfile.
Updated README.md to include developer and testing instructions.
Removed old files and scripts for building and testing.

Signed-off-by: Joris Melchior <jmelchior@pivotal.io>

Dockerfile

@@ -1,5 +1,23 @@
-FROM concourse/buildroot:base
+FROM golang:alpine as builder
+COPY . /go/src/github.com/concourse/github-release-resource
+ENV CGO_ENABLED 0
+ENV GOPATH /go/src/github.com/concourse/github-release-resource/Godeps/_workspace:${GOPATH}
+ENV PATH /go/src/github.com/concourse/github-release-resource/Godeps/_workspace/bin:${PATH}
+RUN go build -o /assets/out github.com/concourse/github-release-resource/cmd/out
+RUN go build -o /assets/in github.com/concourse/github-release-resource/cmd/in
+RUN go build -o /assets/check github.com/concourse/github-release-resource/cmd/check
+RUN set -e; for pkg in $(go list ./...); do \
+		go test -o "/tests/$(basename $pkg).test" -c $pkg; \
+	done
 
-ADD assets/check /opt/resource/check
-ADD assets/in /opt/resource/in
-ADD assets/out /opt/resource/out
+FROM alpine:edge AS resource
+RUN apk add --update bash tzdata
+COPY --from=builder /assets /opt/resource
+
+FROM resource AS tests
+COPY --from=builder /tests /tests
+RUN set -e; for test in /tests/*.test; do \
+		$test; \
+	done
+
+FROM resource

README.md

@@ -124,3 +124,31 @@ prepended with this string. This is useful for adding v in front of version numb
 
 * `globs`: *Optional.* A list of globs for files that will be uploaded alongside
   the created release.
+
+## Development
+
+### Prerequisites
+
+* golang is *required* - version 1.9.x is tested; earlier versions may also
+  work.
+* docker is *required* - version 17.06.x is tested; earlier versions may also
+  work.
+* godep is used for dependency management of the golang packages.
+
+### Running the tests
+
+The tests have been embedded with the `Dockerfile`; ensuring that the testing
+environment is consistent across any `docker` enabled platform. When the docker
+image builds, the test are run inside the docker container, on failure they
+will stop the build.
+
+Run the tests with the following command:
+
+```sh
+docker build -t github-release-resource .
+```
+
+### Contributing
+
+Please make all pull requests to the `master` branch and ensure tests pass
+locally.