.container {
  display: grid;
  grid-template:
    "myHeader myHeader"
    "mySidebar myMainContent"
    "mySidebar myFooter"
    / 1fr 2fr;
  grid-gap: 10px;
}

header { grid-area: myHeader; }
aside { grid-area: mySidebar; }
main { grid-area: myMainContent; }
footer { grid-area: myFooter; }