Add Docker disk usage endpoint at /api/system/docker/disk

[iamkubi]

/api/system/docker/disk endpoint provides disk utilization of the docker daemon. Example output:

$ curl localhost:8080/api/system/docker/disk
{"containers_size": "163.4MB", "images_total":4,"images_active":2,"images_size":"181MB","build_cache_size":0}

[iamkubi]

Added /api/system/docker/image/prune

$ curl -X DELETE -H "Authorization: Bearer ${PELICAN_TOKEN}" localhost:8080/api/system/docker/image/prune
{"ImagesDeleted":[{"Untagged":"ghcr.io/parkervcp/yolks@sha256:bf028a2e41311b30d760a68033e09e9b8258320d10e5639087807f5827474252"},{"Deleted":"sha256:7f9cc1ff412f6c20737d7c796fce106f6d37adaa86260a4f11e7f8017822baa0"},{"Deleted":"sha256:5f5ceb2cd2bc959e2a5458b4d08dfe4681f2eae29caaf655371d377a1a85d82e"},{"Deleted":"sha256:82a9fed77876af1a992506b347530e74b30b467bb39bb1a6185318c1e6ebff08"},{"Deleted":"sha256:82769d9c11f2e7f202c823891f28b006869778ee4a25e3b0797af947de145941"},{"Deleted":"sha256:001ebb1c2b548d928c918cc473aa92cafee19637bcf08cd1cf50fe5dac52640a"},{"Deleted":"sha256:6530bcb1e768ca23eb817d9297227e962627df8a49b720ab6e3516aae361e798"}],
"SpaceReclaimed":61837573}