A Powershell function that wraps Docker command line output to display disk usage. Save the files without the numeric prefix.

1-Get-DockerDiskUsage.ps1

#requires -version 5.1

Function Get-DockerDiskUsage
{
    [cmdletbinding()]
    [alias("gddu")]
    [OutputType("Get-DockerDiskUsage.DockerDiskUsage")]
    Param(
        [Parameter(Position = 0, HelpMessage = "Get specific usage types. The default is All")]
        [ValidateSet("Images", "Containers", "Volumes", "Cache")]
        [string[]]$Type
    )

    Begin
    {
        Write-Verbose "[BEGIN  ] Starting: $($MyInvocation.Mycommand)"

        #define a class
        Class DockerDiskUsage
        {
            [string]$Type
            [int]$Total
            [int]$Active
            [double]$Size
            [double]$Reclaim
            [int]$ReclaimPercent
            #these properties are hidden but they can be used in format.ps1xml files or Select-Object expressions
            hidden [string]$SizeUnit
            hidden [string]$ReclaimUnit

            #methods are hidden because they are only used internally
            hidden [string] GetUnit($value)
            {
                Write-Verbose "[CLASS] Getting unit string from $value"
                [regex]$unitrx = "[KMGT]?B"
                $r = $unitrx.match($value).value
                Write-Verbose "[CLASS] Returning $r"
                return $r
            }

            hidden [double] GetValue($value)
            {
                Write-Verbose "[CLASS] Getting numeric value from $value"
                [regex]$szrx = "\d+(\.\d+)?"
                $r = $szrx.match($value).value
                Write-Verbose "[CLASS] Returning $r"
                return $r
            }

            hidden [int] GetUnitValue([string]$SizeUnit)
            {
                Write-Verbose "[CLASS] Getting unit value from $SizeUnit"
                $unit = Switch ($sizeUnit)
                {
                    "TB" { 1TB; break }
                    "GB" { 1GB; break }
                    "MB" { 1MB; break }
                    "KB" { 1KB; break }
                    default { 1 }
                }
                Write-Verbose "[CLASS] Returning $unit"
                return $unit
            }
            #the constructor
            DockerDiskUsage ($Item)
            {
                Write-Verbose "[CLASS] Invoking constructor for $($item.type)"
                $this.Type = $item.Type
                $this.Total = $item.totalCount
                $this.Active = $item.active
                $this.SizeUnit = $this.getunit($item.size)
                $this.reclaimUnit = $this.GetUnit($item.reclaimable)
                $this.Size = $this.getValue($item.size) * $this.GetUnitValue($this.SizeUnit)
                $this.Reclaim = $this.GetValue($item.reclaimable.split()[0]) * $this.GetUnitValue($this.ReclaimUnit)
                $this.ReclaimPercent = $this.GetValue($item.reclaimable.split()[1])
            }
        } #close class definition

    } #begin
    Process
    {
        Write-Verbose "[PROCESS] Getting Docker Disk Usage"
        $data = docker system df --format "{{json .}}" | ConvertFrom-Json

        if ($data)
        {
            foreach ($item in $data)
            {
                $item | Out-String | Write-Verbose
                #filter on type if specified via the Type parameter
                if ( (-not $Type) -OR ( $item.type -match $($type -join "|")))
                {
                    New-Object -TypeName DockerDiskUsage -ArgumentList $item
                }
            } #foreach item
        }
        else
        {
            Write-Warning "Failed to get any Docker disk usage data"
        }
    } #process
    End
    {
        Write-Verbose "[END    ] Ending: $($MyInvocation.Mycommand)"
    } #end
}

$format = "$PSSCRIPTROOT\docker.diskusage.format.ps1xml"
if (Test-Path -path $format)
{
    Update-FormatData $format
}

2-docker.diskusage.format.ps1xml

<?xml version="1.0" encoding="UTF-8"?>
<!--
format type data generated 04/05/2019 11:19:27
by BOVINE320\Jeff
-->
<Configuration>
  <ViewDefinitions>
    <View>
      <!--Created 04/05/2019 11:19:27 by BOVINE320\Jeff-->
      <Name>default</Name>
      <ViewSelectedBy>
        <TypeName>Get-DockerDiskUsage.DockerDiskUsage</TypeName>
      </ViewSelectedBy>
      <TableControl>
        <TableHeaders>
          <TableColumnHeader>
            <Label>Type</Label>
            <Width>15</Width>
            <Alignment>left</Alignment>
          </TableColumnHeader>
          <TableColumnHeader>
            <Label>Total</Label>
            <Width>7</Width>
            <Alignment>right</Alignment>
          </TableColumnHeader>
          <TableColumnHeader>
            <Label>Active</Label>
            <Width>7</Width>
            <Alignment>right</Alignment>
          </TableColumnHeader>
          <TableColumnHeader>
            <Label>Size</Label>
            <Width>17</Width>
            <Alignment>right</Alignment>
          </TableColumnHeader>
          <TableColumnHeader>
            <Label>Reclaim</Label>
            <Width>14</Width>
            <Alignment>right</Alignment>
          </TableColumnHeader>
          <TableColumnHeader>
            <Label>ReclaimPercent</Label>
            <Width>14</Width>
            <Alignment>right</Alignment>
          </TableColumnHeader>
        </TableHeaders>
        <TableRowEntries>
          <TableRowEntry>
            <TableColumnItems>
              <TableColumnItem>
                <PropertyName>Type</PropertyName>
              </TableColumnItem>
              <TableColumnItem>
                <PropertyName>Total</PropertyName>
              </TableColumnItem>
              <TableColumnItem>
                <PropertyName>Active</PropertyName>
              </TableColumnItem>
              <TableColumnItem>
                <PropertyName>Size</PropertyName>
              </TableColumnItem>
              <TableColumnItem>
                <PropertyName>Reclaim</PropertyName>
              </TableColumnItem>
              <TableColumnItem>
                <PropertyName>ReclaimPercent</PropertyName>
              </TableColumnItem>
            </TableColumnItems>
          </TableRowEntry>
        </TableRowEntries>
      </TableControl>
    </View>
    <View>
      <!--Created 04/05/2019 13:41:17 by BOVINE320\Jeff-->
      <Name>docker</Name>
      <ViewSelectedBy>
        <TypeName>Get-DockerDiskUsage.DockerDiskUsage</TypeName>
      </ViewSelectedBy>
      <TableControl>
        <TableHeaders>
          <TableColumnHeader>
            <Label>Type</Label>
            <Width>15</Width>
            <Alignment>left</Alignment>
          </TableColumnHeader>
          <TableColumnHeader>
            <Label>Total</Label>
            <Width>7</Width>
            <Alignment>left</Alignment>
          </TableColumnHeader>
          <TableColumnHeader>
            <Label>Active</Label>
            <Width>7</Width>
            <Alignment>left</Alignment>
          </TableColumnHeader>
          <TableColumnHeader>
            <Label>Size</Label>
            <Width>11</Width>
            <Alignment>right</Alignment>
          </TableColumnHeader>
          <TableColumnHeader>
            <Label>Reclaimable</Label>
            <Width>16</Width>
            <Alignment>right</Alignment>
          </TableColumnHeader>
        </TableHeaders>
        <TableRowEntries>
          <TableRowEntry>
            <TableColumnItems>
              <TableColumnItem>
                <PropertyName>Type</PropertyName>
              </TableColumnItem>
              <TableColumnItem>
                <PropertyName>Total</PropertyName>
              </TableColumnItem>
              <TableColumnItem>
                <PropertyName>Active</PropertyName>
              </TableColumnItem>
              <TableColumnItem>
                <Scriptblock>"{0}{1}" -f $($_.size/$_.GetUnitValue($_.sizeunit)),$_.sizeunit</Scriptblock>
              </TableColumnItem>
              <TableColumnItem>
                <Scriptblock>"{0}{1} ({2}%)" -f $($_.reclaim/$_.GetUnitValue($_.reclaimunit)),$_.reclaimunit,($_.ReclaimPercent)</Scriptblock>
              </TableColumnItem>
            </TableColumnItems>
          </TableRowEntry>
        </TableRowEntries>
      </TableControl>
    </View>
  </ViewDefinitions>
</Configuration>

Just place these two files in the same directory and without the numeric prefix in the filename.
f.e.
C:\scripts\Get-DockerDiskUsage.ps1 and C:\scripts\docker.diskusage.format.ps1xml