frontend: add BANNER_COLOR to control jumbotron bg

Signed-off-by: Ahmet Alp Balkan <ahmetb@google.com>

src/frontend/handlers.go

@@ -5,6 +5,7 @@ import (
 	"frontend/money"
 	"html/template"
 	"net/http"
+	"os"
 	"strconv"
 	"time"
 
@@ -62,6 +63,7 @@ func (fe *frontendServer) homeHandler(w http.ResponseWriter, r *http.Request) {
 		"currencies":    currencies,
 		"products":      ps,
 		"cart_size":     len(cart),
+		"banner_color":  os.Getenv("BANNER_COLOR"), // illustrates canary deployments
 	}); err != nil {
 		log.Error(err)
 	}

src/frontend/templates/home.html

@@ -2,7 +2,11 @@
 
     {{ template "header" . }}
     <main role="main">
-        <section class="jumbotron text-center mb-0">
+        <section class="jumbotron text-center mb-0"
+		 {{ with $.banner_color }}
+		     style="background-color: {{.}};"
+		 {{ end }}
+		>
             <div class="container">
                 <h1 class="jumbotron-heading">
                     One-stop for Hipster Fashion &amp; Style Online