feat: allow extending validation using project Makefile

(See)[./adrs/0001-extending-validation.md] for why.

Summary:
Allow projects to define their own Makefile target `validate`.
If the Makefile exists in the project using the devtool,
then after devtool's validate has run, run the project's
validate target.

Author: arunpoudel

catalog-info.yaml

@@ -6,6 +6,7 @@ metadata:
   description: Devtools and other useful container images
   annotations:
     backstage.io/techdocs-ref: dir:.
+    backstage.io/adr-location: docs/adrs
     github.com/project-slug: coopnorge/engineering-docker-images
 spec:
   owner: engineering

docs/adrs/0001-extending-validation.md

@@ -0,0 +1,43 @@
+# Allowing extending validation using project Makefile
+
+- Status: draft
+- Decider:
+  - Arun Poudel <[email protected]>
+- Data 2023-12-05
+
+## Context and Problem Statement
+
+Some projects want more validation than what is provided by the default
+by devtools image.
+
+For example: a project using `dbt` would want to run `dbt test` as part of
+the validation process.
+
+The current implementation limits us to only run validations defined in
+specific devtool's Makefile.
+
+## Decision Drivers
+
+- We want to allow projects to define their own validation steps in addition
+  to the ones provided by the devtools image.
+
+## Considered Options
+
+- Allow projects to define their own validation steps in their Makefile.
+- Embed those validations in devtools itself.
+
+## Decision Outcome
+
+Chosen option: "Allow projects to define their own validation steps in their
+Makefile", because:
+
+- It is easier to maintain and extend.
+- It is easier to understand.
+- It allows projects to define their own validation steps without having to
+make upstream contributions to devtools which might not be relevant to other
+projects.
+
+The way this works is, validate first runs the validations defined in the
+devtools, and after that if Makefile exists in the project root, it runs
+the `validate` target defined in the Makefile. If the target returns an error
+(exit code != 0, this might happen when the target is missing as well), the validation fails.