Entendendo melhor a sua imagem

$ docker image ls
REPOSITORY              TAG       IMAGE ID       CREATED          SIZE
conversao-temperatura   latest    ad6bf598faaf   11 minutes ago   377MB
ubuntu                  latest    35a88802559d   2 months ago     78.1MB

A imagem conversao-temperatura possui tamanho grande.

Verificar o histórico de construção de uma imagem:

$ docker image history ubuntu
IMAGE          CREATED        CREATED BY                                      SIZE      COMMENT
35a88802559d   2 months ago   /bin/sh -c #(nop)  CMD ["/bin/bash"]            0B
<missing>      2 months ago   /bin/sh -c #(nop) ADD file:5601f441718b0d192…   78.1MB
<missing>      2 months ago   /bin/sh -c #(nop)  LABEL org.opencontainers.…   0B
<missing>      2 months ago   /bin/sh -c #(nop)  LABEL org.opencontainers.…   0B
<missing>      2 months ago   /bin/sh -c #(nop)  ARG LAUNCHPAD_BUILD_ARCH     0B
<missing>      2 months ago   /bin/sh -c #(nop)  ARG RELEASE                  0B

A imagem conversao-temperatura herda as camadas do ubuntu e adiciona uma camada referente ao container origem:

$ docker image history conversao-temperatura
IMAGE          CREATED          CREATED BY                                      SIZE      COMMENT
ad6bf598faaf   14 minutes ago   /bin/bash                                       299MB
35a88802559d   2 months ago     /bin/sh -c #(nop)  CMD ["/bin/bash"]            0B
<missing>      2 months ago     /bin/sh -c #(nop) ADD file:5601f441718b0d192…   78.1MB
<missing>      2 months ago     /bin/sh -c #(nop)  LABEL org.opencontainers.…   0B
<missing>      2 months ago     /bin/sh -c #(nop)  LABEL org.opencontainers.…   0B
<missing>      2 months ago     /bin/sh -c #(nop)  ARG LAUNCHPAD_BUILD_ARCH     0B
<missing>      2 months ago     /bin/sh -c #(nop)  ARG RELEASE                  0B

$ docker image inspect conversao-temperatura

# ou

$ docker image ls
REPOSITORY              TAG       IMAGE ID       CREATED          SIZE
conversao-temperatura   latest    ad6bf598faaf   17 minutes ago   377MB
ubuntu                  latest    35a88802559d   2 months ago     78.1MB

$ docker image inspect ad6bf598faaf

$ docker image inspect ad6bf598faaf > imagem.json
$ code .

...
        "GraphDriver": {
            "Data": {
                "LowerDir": "/var/lib/docker/overlay2/2e0b8ad43f2a5b202d06d71772dd10181649b11df3ccb81b1970903e10b97f53/diff",
                "MergedDir": "/var/lib/docker/overlay2/04358a8d4bf98f96c7383e4361250c9f58be34cbb7b1ec9c08dcd201a2de1b01/merged",
                "UpperDir": "/var/lib/docker/overlay2/04358a8d4bf98f96c7383e4361250c9f58be34cbb7b1ec9c08dcd201a2de1b01/diff",
                "WorkDir": "/var/lib/docker/overlay2/04358a8d4bf98f96c7383e4361250c9f58be34cbb7b1ec9c08dcd201a2de1b01/work"
            },
            "Name": "overlay2"
        },
...

Filesystem do Ubuntu (lowerdir):

$ sudo su
[sudo] password for marcelo:
root@Latitude5440:/tmp/devops-conversao-temperatura/src# cd /var/lib/docker/overlay2/2e0b8ad43f2a5b202d06d71772dd10181649b11df3ccb81b1970903e10b97f53/diff
root@Latitude5440:/var/lib/docker/overlay2/2e0b8ad43f2a5b202d06d71772dd10181649b11df3ccb81b1970903e10b97f53/diff# ls
bin  boot  dev  etc  home  lib  lib64  media  mnt  opt  proc  root  run  sbin  srv  sys  tmp  usr  var

Filesystem adicionado (upperdir):

$ cd /var/lib/docker/overlay2/04358a8d4bf98f96c7383e4361250c9f58be34cbb7b1ec9c08dcd201a2de1b01/diff
root@Latitude5440:/var/lib/docker/overlay2/04358a8d4bf98f96c7383e4361250c9f58be34cbb7b1ec9c08dcd201a2de1b01/diff# ls
app  etc  nodesource_setup.sh  root  tmp  usr  var