Environtment Replacement

Hai semuanya, di materi kali ini kita akan membahas lebih dalam tentang Env pada Dockerfile diantaranya seperti berikut:

Environtment Variable
Build Args
Clean

Ok kita langsung ja ke pembahasan yang pertama yaitu environtment variable

Environtment Variable

Environment variables (declared with the ENV statement) can also be used in certain instructions as variables to be interpreted by the Dockerfile. Escapes are also handled for including variable-like syntax into a statement literally.

Environment variables are notated in the Dockerfile either with $variable_name or ${variable_name}. They are treated equivalently and the brace syntax is typically used to address issues with variable names with no whitespace, like ${foo}_bar.

The ${variable_name} syntax also supports a few of the standard bash modifiers as specified below:

${variable:-word} indicates that if variable is set then the result will be that value. If variable is not set then word will be the result.
${variable:+word} indicates that if variable is set then word will be the result, otherwise the result is the empty string.

Environment variables are supported by the following list of instructions in the Dockerfile: ADD, COPY, ENV, EXPOSE, FROM, LABEL, STOPSIGNAL, USER, VOLUME, WORKDIR, and ONBUILD

Contoh penggunaanya seperti berikut:

Jika di jalankan maka hasilnya seperti berikut:

➜ 07-dockerfile  docker build -t dimmaryanto93/centos:0.2 .
[+] Building 0.1s (6/6) FINISHED
 => [internal] load build definition from Dockerfile                                                                   0.0s 
 => => transferring dockerfile: 281B                                                                                   0.0s 
 => [internal] load .dockerignore                                                                                      0.0s 
 => => transferring context: 2B                                                                                        0.0s 
 => [internal] load metadata for docker.io/library/centos:7                                                            0.0s 
 => [1/2] FROM docker.io/library/centos:7                                                                              0.0s 
 => CACHED [2/2] RUN mkdir -p /usr/share/nginx/html && echo "<html><head><title>Halo World</title></head><body><h3>it"  0.0s 
 => exporting to image                                                                                                 0.0s 
 => => exporting layers                                                                                                0.0s 
 => => writing image sha256:22b22460babc563c047eed204534a463d34a13a1cb75dda635d49bcddcb9351e                           0.0s 
 => => naming to docker.io/dimmaryanto93/centos:0.2

➜ 07-dockerfile  docker run --rm dimmaryanto93/centos:0.2  
<html><head><title>Halo World</title></head><body><h3>it''s Works!</h3></body></html>

Build Args

The ARG instruction defines a variable that users can pass at build-time to the builder with the docker build command using the --build-arg <varname>=<value> flag. If a user specifies a build argument that was not defined in the Dockerfile, the build outputs a warning.

A Dockerfile may include one or more ARG instructions. For example, the following is a valid Dockerfile:

ARG <name>[=<default value>]

Notes: It is not recommended to use build-time variables for passing secrets like github keys, user credentials etc. Build-time variable values are visible to any user of the image with the docker history command. Refer to the “build images with BuildKit” section to learn about secure ways to use secrets when building images.

Berikut adalah contoh penggunaanya:

Jika dijalankan maka hasilnya seperti berikut:

➜ 07-dockerfile  docker build -t dimmaryanto93/centos:0.2 .
[+] Building 0.4s (6/6) FINISHED
 => [internal] load build definition from Dockerfile                                   0.0s
 => => transferring dockerfile: 365B                                                   0.0s
 => [internal] load .dockerignore                                                      0.0s
 => => transferring context: 173B                                                      0.0s
 => [internal] load metadata for docker.io/library/centos:7                            0.0s
 => CACHED [1/2] FROM docker.io/library/centos:7                                       0.0s
 => [2/2] RUN mkdir -p /usr/share/nginx/html && echo "<html><head><title>Halo World</"  0.2s
 => exporting to image                                                                 0.0s
 => => exporting layers                                                                0.0s
 => => writing image sha256:d6171a364212fcba9782882335591e02c650c52edef49c511878ed0a7  0.0s
 => => naming to docker.io/dimmaryanto93/centos:0.2

➜ 07-dockerfile  docker run --rm dimmaryanto93/centos:0.2
<html><head><title>Halo World</title></head><body><h3>it''s Works!</h3></body></html>

## Override default value
➜ 07-dockerfile  docker build --build-arg IMAGE_VERSION=8 -t dimmaryanto93/centos:0.2 .
[+] Building 29.3s (7/7) FINISHED
 => [internal] load build definition from Dockerfile                                   0.0s
 => => transferring dockerfile: 32B                                                    0.0s
 => [internal] load .dockerignore                                                      0.0s
 => => transferring context: 35B                                                       0.0s
 => [internal] load metadata for docker.io/library/centos:8                            4.1s
 => [auth] library/centos:pull token for registry-1.docker.io                          0.0s
 => [1/2] FROM docker.io/library/centos:8@sha256:5528e8b1b1719d34604c87e11dcd1c0a20b  24.5s
 => => resolve docker.io/library/centos:8@sha256:5528e8b1b1719d34604c87e11dcd1c0a20be  0.0s
 => => sha256:7a0437f04f83f084b7ed68ad9c4a4947e12fc4e1b006b38129ba 75.18MB / 75.18MB  21.8s
 => => sha256:5528e8b1b1719d34604c87e11dcd1c0a20bedf46e83b5632cdeac91b8c0 762B / 762B  0.0s
 => => sha256:dbbacecc49b088458781c16f3775f2a2ec7521079034a7ba499c8b0bb7f 529B / 529B  0.0s
 => => sha256:300e315adb2f96afe5f0b2780b87f28ae95231fe3bdd1e16b9ba606 2.14kB / 2.14kB  0.0s
 => => extracting sha256:7a0437f04f83f084b7ed68ad9c4a4947e12fc4e1b006b38129bac89114ec  2.5s
 => [2/2] RUN mkdir -p /usr/share/nginx/html && echo "<html><head><title>Halo World</"  0.6s
 => exporting to image                                                                 0.0s
 => => exporting layers                                                                0.0s
 => => writing image sha256:2ecf53b20c0c7cbe588a3d1142f0cf58aafe7edacc66fe74d906d9b71  0.0s
 => => naming to docker.io/dimmaryanto93/centos:0.2

➜ 07-dockerfile  docker run --rm dimmaryanto93/centos:0.2
<html><head><title>Halo World</title></head><body><h3>it''s Works!</h3></body></html>

Selain itu juga, kita disediakan Predefined ARGs yaitu HTTP_PROXY, http_proxy, HTTPS_PROXY, https_proxy, FTP_PROXY, ftp_proxy, NO_PROXY, dan no_proxy.