Update building stuff

- Add a Makefile with common targets (binary, test, validate, ..)
- Use docker to build it (isolated)

And update circleci to work with the new Makefile..

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

script/validate-gofmt

@@ -0,0 +1,30 @@
+#!/bin/bash
+
+source "$(dirname "$BASH_SOURCE")/.validate"
+
+IFS=$'\n'
+files=( $(validate_diff --diff-filter=ACMR --name-only -- '*.go' || true) )
+unset IFS
+
+badFiles=()
+for f in "${files[@]}"; do
+	# we use "git show" here to validate that what's committed is formatted
+	if [ "$(git show "$VALIDATE_HEAD:$f" | gofmt -s -l)" ]; then
+		badFiles+=( "$f" )
+	fi
+done
+
+if [ ${#badFiles[@]} -eq 0 ]; then
+	echo 'Congratulations!  All Go source files are properly formatted.'
+else
+	{
+		echo "These files are not properly gofmt'd:"
+		for f in "${badFiles[@]}"; do
+			echo " - $f"
+		done
+		echo
+		echo 'Please reformat the above files using "gofmt -s -w" and commit the result.'
+		echo
+	} >&2
+	false
+fi