use --port from PORT env var

Signed-off-by: Sebastian Webber <sebastian@swebber.me>

Author: sebastianwebber

cmd/api/Dockerfile

@@ -1,10 +1,12 @@
 FROM alpine:3.13 as builder
 FROM scratch
     EXPOSE 3000
+    ENV PORT=3000
 
     COPY --from=builder /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/
 
     WORKDIR /app
     COPY api /app
     COPY *.yml /app
-    CMD ["/app/api"]
+
+    ENTRYPOINT [ "/app/api" ]

cmd/api/main.go

@@ -4,6 +4,8 @@ import (
 	"flag"
 	"fmt"
 	"log"
+	"os"
+	"strconv"
 
 	_ "github.com/pgconfig/api/cmd/api/docs" // docs is generated by Swag CLI, you have to import it.
 	v1 "github.com/pgconfig/api/cmd/api/handlers/v1"
@@ -18,8 +20,20 @@ var (
 	docsFile  string
 )
 
+func getDefaultPort(envName string) int {
+	defaultPort := 3000
+	val := os.Getenv(envName)
+
+	out, err := strconv.Atoi(val)
+	if err != nil {
+		return defaultPort
+	}
+
+	return out
+}
+
 func init() {
-	flag.IntVar(&port, "port", 3000, "Listen port")
+	flag.IntVar(&port, "port", getDefaultPort("PORT"), "Listen port")
 	flag.StringVar(&rulesFile, "rules-file", "./rules.yml", "Rules file")
 	flag.StringVar(&docsFile, "docs-file", "./pg-docs.yml", "Rules file")
 	flag.Parse()