Rebuilding Docker image taking time

Question

Dockerfile has the following content,

FROM node:16.4.2-alpine3.14

WORKDIR /app

COPY package.json .

COPY . /app

And ran the following build command,

docker build -t app:0.1 .

It took 28.4 seconds and below is the terminal logs,

[+] Building 28.4s (10/10) FINISHED
 => [internal] load build definition from Dockerfile                                                                                                     0.1s
 => => transferring dockerfile: 124B                                                                                                                     0.0s
 => [internal] load .dockerignore                                                                                                                        0.1s
 => => transferring context: 53B                                                                                                                         0.0s
 => [internal] load metadata for docker.io/library/node:16.4.2-alpine3.14                                                                               17.1s
 => [auth] library/node:pull token for registry-1.docker.io                                                                                              0.0s
 => [internal] load build context                                                                                                                        0.1s
 => => transferring context: 2.01MB                                                                                                                      0.0s
 => [1/4] FROM docker.io/library/node:16.4.2-alpine3.14@sha256:fabfca5e7dcb339097f998d6ef11c53dd80a3f99ed5cecc005e93d0ff6d4bda9                          9.9s
 => => resolve docker.io/library/node:16.4.2-alpine3.14@sha256:fabfca5e7dcb339097f998d6ef11c53dd80a3f99ed5cecc005e93d0ff6d4bda9                          0.0s
 => => sha256:fabfca5e7dcb339097f998d6ef11c53dd80a3f99ed5cecc005e93d0ff6d4bda9 1.00kB / 1.00kB                                                           0.0s
 => => sha256:75dec02064547a8ec570f2953e8d68a1674ad3f37730160f1570cce077be9ed0 1.16kB / 1.16kB                                                           0.0s
 => => sha256:40cb916373b08a087466d2e72402d0b3a4587fd3e9135169498cf0db4ff42a88 6.53kB / 6.53kB                                                           0.0s
 => => sha256:5843afab387455b37944e709ee8c78d7520df80f8d01cf7f861aae63beeddb6b 2.81MB / 2.81MB                                                           0.8s
 => => sha256:c118dce16b0057d713fc98e31606a84e4348fa2c967eaf1bb5fd21ba42825956 35.55MB / 35.55MB                                                         7.1s
 => => sha256:aef8e8137ac43c8199343c96874993063af6584260f22b15e99f735cce5de653 2.35MB / 2.35MB                                                           2.6s
 => => extracting sha256:5843afab387455b37944e709ee8c78d7520df80f8d01cf7f861aae63beeddb6b                                                                0.2s
 => => sha256:ad336e0e52b8dfc38c23599663deb060b1ac169d548dec8072ead94712f708be 281B / 281B                                                               2.0s
 => => extracting sha256:c118dce16b0057d713fc98e31606a84e4348fa2c967eaf1bb5fd21ba42825956                                                                2.0s
 => => extracting sha256:aef8e8137ac43c8199343c96874993063af6584260f22b15e99f735cce5de653                                                                0.2s
 => => extracting sha256:ad336e0e52b8dfc38c23599663deb060b1ac169d548dec8072ead94712f708be                                                                0.0s
 => [2/4] WORKDIR /app                                                                                                                                   0.6s
 => [3/4] COPY package.json .                                                                                                                            0.1s
 => [4/4] COPY . /app                                                                                                                                    0.1s
 => exporting to image                                                                                                                                   0.2s
 => => exporting layers                                                                                                                                  0.2s
 => => writing image sha256:91d93eddff55cba6bd8b72144b7320e025de93e9865177ff584c75b94d1bafc1                                                             0.0s
 => => naming to docker.io/library/app:0.1

When I run the same build command again, it is taking 14.6 seconds.

However if I pull the node:16.4.2-alpine3.14 using,

 docker pull node:16.4.2-alpine3.14

and then run the build command, then build takes only 0.3 seconds

I think, when we build an image, dependencies also get downloaded and that is why the time taken reduces from 28.4 to 14.6 seconds. But why even 14.6 seconds? It should be as less as 0.3 seconds.

Why is this so? What am I missing?

Answer 1

The following could be the reasons it takes 18.4 seconds:

1. Docker client takes the entire build context to the docker daemon.The build context is the entire directory the Dockerfile.some files or folders can take up a lot of space e.g node_modules. A remedy to this is to add the file that is not required eg .git, node_module, log files to the .dockerignore file to get Docker to ignore some files.
2. DNS resolution. Check how long it takes to resolve the docker registry using the dig command.
3. Enabling the buildkit for your build will help with improving the build time. Setting the DOCKER_BUILDKIT=1 environment variable when invoking the docker build command such as:

DOCKER_BUILDKIT=1 docker build .

please look at this link for more information