validate: add yamllint validation

validate other YAML files, such as the ones used in the documentation,
and GitHub actions workflows, to prevent issues such as;

- 30295c1750714d26f3c8fc9c3451f11ac351f2be
- 8e8d9a36500fb07fa9d1b68539756b9a93d3509e

With this patch:

    hack/validate/yamllint
    Congratulations! yamllint config file formatted correctly
    Congratulations! YAML files are formatted correctly

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

hack/validate/default

@@ -8,6 +8,7 @@ export SCRIPTDIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 #. "${SCRIPTDIR}"/dco
 . "${SCRIPTDIR}"/default-seccomp
 . "${SCRIPTDIR}"/pkg-imports
+. "${SCRIPTDIR}"/yamllint
 . "${SCRIPTDIR}"/swagger
 . "${SCRIPTDIR}"/swagger-gen
 . "${SCRIPTDIR}"/toml

hack/validate/yamllint

@@ -0,0 +1,29 @@
+#!/usr/bin/env bash
+set -e
+SCRIPTDIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+source "${SCRIPTDIR}/.validate"
+
+if [ -n "${TEST_FORCE_VALIDATE:-}" ]; then
+	files=(docs/api/*.yaml)
+else
+	IFS=$'\n'
+	files=($(validate_diff --diff-filter=ACMR --name-only -- docs/*.yaml || true))
+	unset IFS
+fi
+
+# validate the yamllint configuration file before anything else
+if out=$(yamllint -f parsable -d "{extends: default, rules: {document-start: disable}}" "${SCRIPTDIR}"/yamllint.yaml); then
+	echo "Congratulations! yamllint config file formatted correctly"
+else
+	echo "${out}" >&2
+	false
+fi
+
+# Then validate GitHub actions workflows, and conditionally lint the swagger
+# files in the docs directory, as these are large files and take some time.
+if out=$(yamllint -f parsable -c "${SCRIPTDIR}"/yamllint.yaml .github/workflows/*.yml "${files[@]}"); then
+	echo "Congratulations! YAML files are formatted correctly"
+else
+	echo "${out}" >&2
+	false
+fi