Grid - 1 column and 3 rows

.container {  display: grid;
  grid-template-columns: 1fr;
  grid-template-rows: 6em 1fr 12em;
  gap: 0px 0px;
  grid-auto-flow: row;
  grid-template-areas:
    "header"
    "main"
    "footer";
}

.header { grid-area: header; }
.main { grid-area: main; }
.footer { grid-area: footer; }