fix: add docker entrypoint to set appropriate defaults for env vars

cmd/docker-entrypoint/main.go

@@ -0,0 +1,43 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+)
+
+var dockerEnvVarDefaults = map[string]string{
+	"BACKREST_PORT": "0.0.0.0:9898",
+}
+
+func main() {
+	var defaultedVariables []string
+	for key, value := range dockerEnvVarDefaults {
+		if os.Getenv(key) == "" {
+			os.Setenv(key, value)
+			defaultedVariables = append(defaultedVariables, fmt.Sprintf("%s=%q", key, value))
+		}
+	}
+	if len(defaultedVariables) > 0 {
+		os.Stderr.WriteString("Setting docker defaults for env variables:\n")
+		for _, key := range defaultedVariables {
+			os.Stderr.WriteString(" - " + key + "\n")
+		}
+	}
+	if len(os.Args) < 1 {
+		os.Stderr.WriteString("No command provided to run.\n")
+		os.Exit(1)
+	}
+
+	cmd := exec.Command(os.Args[0], os.Args[1:]...)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	cmd.Stdin = os.Stdin
+
+	if err := cmd.Run(); err != nil {
+		if exitError, ok := err.(*exec.ExitError); ok {
+			os.Exit(exitError.ExitCode())
+		}
+		panic(err)
+	}
+}