Building containers makes lots of different layers. When you make a change to an element in the build all following layers have to be rebuilt because they cannot be taken from cache. A simple example I have used before that has way too many layers:

docker build -t test .
Sending build context to Docker daemon  5.632kB
Step 1/7 : FROM ubuntu:latest
 ---> 7698f282e524
Step 2/7 : RUN echo "Test"
 ---> Using cache
 ---> 75ac3bfbeaba
Step 3/7 : COPY 1 .
 ---> Using cache
 ---> d457a7492d2c
Step 4/7 : ADD 2 .
 ---> Using cache
 ---> 1c3284c1e6a0
Step 5/7 : ADD 3 .
 ---> Using cache
 ---> 96ab91bcf3df
Step 6/7 : ADD 4 .
 ---> Using cache
 ---> 2889643b631b
Step 7/7 : ADD 5 .
 ---> Using cache
 ---> adb6797fe48a
Successfully built adb6797fe48a
Successfully tagged test:latest

If you examine the number of dangling layers (layers not connected to an active image) for my build there are none:

docker images -f dangling=true
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE

Now I am going to modify one of the files that are being added at step 3/7 so it triggers a rebuild:

docker build -t test .
Sending build context to Docker daemon   5.12kB
Step 1/7 : FROM ubuntu:latest
 ---> 7698f282e524
Step 2/7 : RUN echo "Test"
 ---> Using cache
 ---> 75ac3bfbeaba
Step 3/7 : COPY 1 .
 ---> Using cache
 ---> d457a7492d2c
Step 4/7 : ADD 2 .
 ---> a93a35a37ebe
Step 5/7 : ADD 3 .
 ---> b90d01ee0806
Step 6/7 : ADD 4 .
 ---> 43af309b28d8
Step 7/7 : ADD 5 .
 ---> 540007e7c833
Successfully built 540007e7c833
Successfully tagged test:latest

Now if we check for dangling layers we now have layer that was replaced 3/7:

docker images -f dangling=true
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
<none>              <none>              adb6797fe48a        10 minutes ago      69.9MB

You can identify where these dangling layers are stored by doing docker image inspect:

docker image inspect test
[
    {
        "Id": "sha256:540007e7c833d09da0edaad933d4126075bffa32badb1170da363e1e1f220c4c",
        "RepoTags": [
            "test:latest"
        ],
        "RepoDigests": [],
        "Parent": "sha256:43af309b28d81faa983b87d2db2f64b27b7658f93639f10b07d53b50dded7c45",
        "Comment": "",
        "Created": "2019-06-19T02:46:38.5065201Z",
        "Container": "",
        "ContainerConfig": {
            "Hostname": "",
            "Domainname": "",
            "User": "",
            "AttachStdin": false,
            "AttachStdout": false,
            "AttachStderr": false,
            "Tty": false,
            "OpenStdin": false,
            "StdinOnce": false,
            "Env": [
                "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
            ],
            "Cmd": [
                "/bin/sh",
                "-c",
                "#(nop) ADD file:f5b9e73db3de1fef2d430837d0d31af0af9c9405c64349def90530b2fc8ca6d2 in . "
            ],
            "ArgsEscaped": true,
            "Image": "sha256:43af309b28d81faa983b87d2db2f64b27b7658f93639f10b07d53b50dded7c45",
            "Volumes": null,
            "WorkingDir": "",
            "Entrypoint": null,
            "OnBuild": null,
            "Labels": null
        },
        "DockerVersion": "18.09.2",
        "Author": "",
        "Config": {
            "Hostname": "",
            "Domainname": "",
            "User": "",
            "AttachStdin": false,
            "AttachStdout": false,
            "AttachStderr": false,
            "Tty": false,
            "OpenStdin": false,
            "StdinOnce": false,
            "Env": [
                "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
            ],
            "Cmd": [
                "/bin/bash"
            ],
            "ArgsEscaped": true,
            "Image": "sha256:43af309b28d81faa983b87d2db2f64b27b7658f93639f10b07d53b50dded7c45",
            "Volumes": null,
            "WorkingDir": "",
            "Entrypoint": null,
            "OnBuild": null,
            "Labels": null
        },
        "Architecture": "amd64",
        "Os": "linux",
        "Size": 69859108,
        "VirtualSize": 69859108,
        "GraphDriver": {
            "Data": {
                "LowerDir": "/var/lib/docker/overlay2/77fcf68e04407dd297d93203e242e52cff942b2d8508b3b6e4db2f62d53f38bc/diff:/var/lib/docker/overlay2/dd3f244fb32b847a5d8b2b18e219142bbe3b2e61fe107e290c144154b4513d5e/diff:/var/lib/docker/overlay2/839011eb0edb75c6fedb5e4a9155de2e4bd6305d233ed085d40a4e5166328736/diff:/var/lib/docker/overlay2/3dc3a1f4d37b525f672196b55033c6b82e4ddfc20de16aa803c36fe2358bcb32/diff:/var/lib/docker/overlay2/6e11b07d20377b78ee134a037fc6e661364d273d861419eb77126d0d228abbf0/diff:/var/lib/docker/overlay2/f8c5f20e6ebd1ec759101d926a5101a36ef2378af828ef57a0f8e4a8a467f76f/diff:/var/lib/docker/overlay2/77a101af01c69427ced57be20f01d4a6a688ff2b13d50260be7a7fda1bd7fbf5/diff",
                "MergedDir": "/var/lib/docker/overlay2/28f3e078ee98a327274c59c653858559ad81b866a40e62a70cca989ee403f2a6/merged",
                "UpperDir": "/var/lib/docker/overlay2/28f3e078ee98a327274c59c653858559ad81b866a40e62a70cca989ee403f2a6/diff",
                "WorkDir": "/var/lib/docker/overlay2/28f3e078ee98a327274c59c653858559ad81b866a40e62a70cca989ee403f2a6/work"
            },
            "Name": "overlay2"
        },
        "RootFS": {
            "Type": "layers",
            "Layers": [
                "sha256:02571d034293cb241c078d7ecbf7a84b83a5df2508f11a91de26ec38eb6122f1",
                "sha256:270f934787edf0135132b6780cead0f12ca11690c5d6a5d395e44d290912100a",
                "sha256:8d267010480fed7e616b9b7861854042aad4ef5e55f8771f2c738061640d2cb0",
                "sha256:ea9703e9d50c6fdd693103fee05c65e8cc25be44c6e6587dd89c6559d8df2de7",
                "sha256:69d3f4708a57a9355cf65a99274e6b79788a052564c4fb0fd90f5283c109946a",
                "sha256:d18953dc7e1eef0e19b52db05c2ff34089e9f1166766c8f57b8475db5a3c79b8",
                "sha256:f1ce2d9ca96cc9cd13caab945986580eae2404e87d81b1b485b12ee242c37889",
                "sha256:aeb58c1f315c5baacbe4c2db1745dec548753197e2b251a958704addfd33a8c2"
            ]
        },
        "Metadata": {
            "LastTagTime": "2019-06-19T02:46:38.5547836Z"
        }
    }
]

You can see all the layers are stored in /var/lib/docker/overlay2 you can use the dangling layer id to locate how much space is now wasted on your hard drive. You can remove these dangling layers with:

docker rmi $(docker images -f dangling=true)

These dangling images can eat up tons of space on your build machine. So you need to automate the cleanup process to avoid wasting space.