`docker build` show output from `RUN` [duplicate]

Question

I have this Dockerfile, where I run a command with RUN, and I want to see its output when running docker build:

FROM alpine:3.14
COPY . .
RUN echo "here are some numbers: $(seq 10)"

When I run docker build ., it doesn't show the output of the above command:

[+] Building 2.8s (8/8) FINISHED                                                                                                           
 => [internal] load build definition from Dockerfile                                                                                  0.0s
 => => transferring dockerfile: 36B                                                                                                   0.0s
 => [internal] load .dockerignore                                                                                                     0.0s
 => => transferring context: 2B                                                                                                       0.0s
 => [internal] load metadata for docker.io/library/alpine:3.14                                                                        1.2s
 => [internal] load build context                                                                                                     0.1s
 => => transferring context: 188.24kB                                                                                                 0.1s
 => CACHED [1/3] FROM docker.io/library/alpine:3.14@sha256:234cb88d3020898631af0ccbbcca9a66ae7306ecd30c9720690858c1b007d2a0           0.0s
 => [2/3] COPY . .                                                                                                                    0.4s
 => [3/3] RUN echo "here are some numbers: $(seq 10)"                                                                                 0.2s
 => exporting to image                                                                                                                0.8s
 => => exporting layers                                                                                                               0.8s
 => => writing image sha256:7e0bf9ff04a7b4a3a53395d430ddd950500be9a53733cce22a1da1929cae1a0a                                          0.0s 
                                                                                                                                           
Use 'docker scan' to run Snyk tests against images to find vulnerabilities and learn how to fix them.

How do I see what the output is?

Docker version:

$ docker --version
Docker version 20.10.6, build 370c289

Answer 1

Is that what are you looking for?

$ docker build --progress=plain .

Sending build context to Docker daemon  4.096kB
Step 1/3 : FROM alpine:3.14
3.14: Pulling from library/alpine
5843afab3874: Pull complete 
Digest: sha256:234cb88d3020898631af0ccbbcca9a66ae7306ecd30c9720690858c1b007d2a0
Status: Downloaded newer image for alpine:3.14
 ---> d4ff818577bc
Step 2/3 : COPY . .
 ---> 106aa79185ae
Step 3/3 : RUN echo "here are some numbers: $(seq 10)"
 ---> Running in 30a81b6d5035
here are some numbers: 1
2
3
4
5
6
7
8
9
10
Removing intermediate container 30a81b6d5035
 ---> 3c059c9b6150
Successfully built 3c059c9b6150

$ docker --version
Docker version 19.03.8, build afacb8b

Answer 2

Docker is showing the output as it's generated, and then hiding it when the command is done. You can disable this interactive output by piping docker build to a file or pager. Make sure to pipe stderr as well as stdout. For example, in bash, the result of the command appears in item #7 here:

$ docker build . |& cat
#1 [internal] load build definition from Dockerfile
#1 sha256:0ce5eece4291db925ff5f4646c0f72c5c94a28a77431c2afd38ffa4c5a90ccab
#1 transferring dockerfile: 36B done
#1 DONE 0.0s

#2 [internal] load .dockerignore
#2 sha256:96c5d624b98a42c7926385cf683312588be6b5b47ce4512bbc5a0a59c4042367
#2 transferring context: 2B done
#2 DONE 0.0s

#3 [internal] load metadata for docker.io/library/alpine:3.14
#3 sha256:af035606328a1ce217c0e290353f888d2ee03ed437bef601e11d9cc421fbbb67
#3 DONE 0.9s

#4 [1/3] FROM docker.io/library/alpine:3.14@sha256:234cb88d3020898631af0ccbbcca9a66ae7306ecd30c9720690858c1b007d2a0
#4 sha256:2c66994e2bde1811eaa777244107d7b0d683d113e151d83a5f7aa09e37e1883f
#4 DONE 0.0s

#5 [internal] load build context
#5 sha256:c87370e17d42a78ca943cd74a0d2d983bc71dc609a92739af5013ed4373e31a6
#5 transferring context: 188.24kB 0.1s done
#5 DONE 0.1s

#4 [1/3] FROM docker.io/library/alpine:3.14@sha256:234cb88d3020898631af0ccbbcca9a66ae7306ecd30c9720690858c1b007d2a0
#4 sha256:2c66994e2bde1811eaa777244107d7b0d683d113e151d83a5f7aa09e37e1883f
#4 CACHED

#6 [2/3] COPY . .
#6 sha256:200618f800c74119b8db3b01ff18105a07477dcfae41f0528189443421f6ba31
#6 DONE 0.4s

#7 [3/3] RUN echo "here are some numbers: $(seq 10)"
#7 sha256:c094a42d846e25e5586053168f7231e17248fc7f09c75de1199642ad83688952
#7 0.241 here are some numbers: 1
#7 0.241 2
#7 0.241 3
#7 0.241 4
#7 0.241 5
#7 0.241 6
#7 0.241 7
#7 0.241 8
#7 0.241 9
#7 0.241 10
#7 DONE 0.3s

#8 exporting to image
#8 sha256:e8c613e07b0b7ff33893b694f7759a10d42e180f2b4dc349fb57dc6b71dcab00
#8 exporting layers
#8 exporting layers 0.8s done
#8 writing image sha256:7b4e8d44804bba048cf5aa47c58cd57ab701400143177fb63887ef51edc1a23b done
#8 DONE 0.8s

Use 'docker scan' to run Snyk tests against images to find vulnerabilities and learn how to fix them