How to get a vuetify data table to span the entire width of a container when it is inside a card?

Instead of the <template>, wrap the card in a <v-layout child-flex> like so:

...
<v-content>
  <v-container fluid fill-height>

    <v-layout child-flex>
      <v-card>
        <v-card-title>
          ...

child-flex is a class that gives its children (in this case just the card) the css rule flex: 1 1 auto, making them grow to fill available space.