ci: build syn2mas docker image as part of the build.yaml workflow

docker-bake.hcl

@@ -1,9 +1,10 @@
 // This is what is baked by GitHub Actions
-group "default" { targets = ["regular", "debug"] }
+group "default" { targets = ["regular", "debug", "syn2mas"] }
 
 // Targets filled by GitHub Actions: one for the regular tag, one for the debug tag
 target "docker-metadata-action" {}
 target "docker-metadata-action-debug" {}
+target "docker-metadata-action-syn2mas" {}
 
 // This sets the platforms and is further extended by GitHub Actions to set the
 // output and the cache locations
@@ -22,3 +23,8 @@ target "debug" {
   inherits = ["base", "docker-metadata-action-debug"]
   target = "debug"
 }
+
+target "syn2mas" {
+  inherits = ["base", "docker-metadata-action-syn2mas"]
+  context = "./tools/syn2mas"
+}