feat(Docker): create development Docker image

- Create Docker image to be used for experimentation during development
- Create targets in `Makefile` pertaining to the above

Dockerfile

@@ -0,0 +1,8 @@
+# NB: this image isn't used anywhere in the build pipeline. It exists to
+# conveniently facilitate ad-hoc experimentation in a sandboxed environment
+# during development.
+FROM golang:1.15-alpine
+
+RUN apk add git less make
+
+WORKDIR /app