Prepare for dependency vendoring.

- Add helper script to simplify glide usage.
- Add validation script for unwanted changes to vendoring.
- Relax/tighten up .{git,docker}ignore to cover vendored files properly.
- .validate: Protect from unbound variable in case of nounset setting.
- Install more recent hg version in the build container.
- Remove glide installation steps from Dockerfile.
- Update documentation.

script/validate-vendor

@@ -0,0 +1,37 @@
+#!/bin/bash
+set -o errexit
+set -o pipefail
+set -o nounset
+
+SCRIPTDIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"; export SCRIPTDIR
+source "${SCRIPTDIR}/.validate"
+
+# Iterate over all directories containing vendor folders.
+for dir in . integration; do
+	vendor_dir="${dir}/vendor/"
+	IFS=$'\n' files=( $(validate_diff --diff-filter=ACMR --name-only -- ${vendor_dir} || true) )
+
+	if [[ ${#files[@]} -gt 0 ]]; then
+		# We run glide install to and see if we have a diff afterwards
+		echo "checking ${vendor_dir} for unintentional changes..."
+		(
+		cd ${dir}
+		"${SCRIPTDIR}/glide.sh" install
+		)
+		# Let see if the working directory is clean
+		diffs="$(git status --porcelain -- ${vendor_dir} 2>/dev/null)"
+		if [[ "$diffs" ]]; then
+			{
+				echo "The result of 'glide install' for vendor directory '${dir}' differs"
+				echo
+				echo "$diffs"
+				echo
+				echo 'Please vendor your package(s) with script/glide.sh.'
+				echo
+			} >&2
+			exit 2
+		fi
+	fi
+done
+
+echo 'Congratulations! All vendoring changes are done the right way.'