Clean up unused Docker images #14956

Job	Run time
raspi-2 / initialize	48s
raspi-2-skia / initialize	51s
raspi-2 / docker-build-image	26m 22s
raspi-2 / docker-unittest-image	0s
raspi-2-skia / docker-build-image	25m 30s
raspi-2-skia / docker-unittest-image	0s
raspi-2-skia / raspi-2-skia_devel	10m 14s
raspi-2-skia / raspi-2-skia_debug	14m 6s
raspi-2-skia / raspi-2-skia_qa	9m 3s
raspi-2-skia / raspi-2-skia_gold	8m 29s
raspi-2 / raspi_devel	10m 9s
raspi-2 / raspi_debug	14m 45s
raspi-2 / raspi_qa	9m 57s
raspi-2 / raspi_gold	9m 44s
raspi-2 / sbversion-15_devel	9m 58s
raspi-2 / sbversion-15_debug	13m 54s
raspi-2 / sbversion-15_qa	9m 9s
raspi-2 / sbversion-15_gold	9m 5s
raspi-2 / sbversion-16_devel	10m 11s
raspi-2 / sbversion-16_debug	15m 0s
raspi-2 / sbversion-16_qa	9m 3s
raspi-2 / sbversion-16_gold	8m 30s
raspi-2 / sbversion-17_devel	10m 10s
raspi-2 / sbversion-17_debug	14m 26s
raspi-2 / sbversion-17_qa	9m 14s
raspi-2 / sbversion-17_gold	8m 58s
raspi-2-skia / ${{ matrix.name }}_${{ matrix.shard }}_test	0s
raspi-2-skia / ${{ matrix.name }}_on_device_${{ matrix.shard }}	0s
raspi-2 / ${{ matrix.name }}_${{ matrix.shard }}_test	0s
raspi-2 / ${{ matrix.name }}_on_device_${{ matrix.shard }}	0s
	4h 27m 36s

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