Add env vars section

Author: maricaantonacci

docs/container/env_vars.md

@@ -0,0 +1,70 @@
+An environment variable consists of a variable name and its value.
+
+There are two ways to set environment variables for a docker container: with CLI arguments, using an _env_ file.
+
+### CLI arguments
+
+When we launch our Docker container, **we can pass environment variables as key-value pairs directly into the command line** using the parameter _–env_ (or its short form _-e_).
+
+For instance, let's execute the following command:
+
+```
+$ docker run --rm --env VARIABLE1=foobar alpine env
+```
+
+The environment variables we set will be printed to the console:
+
+```
+VARIABLE1=foobar
+```
+
+As can be seen, the Docker container correctly interprets the variable `VARIABLE1`.
+
+Also, **we can omit the value in the command line if the variable already exists in the local environment**.
+
+For example, let's define a local environment variable:
+
+```
+export VARIABLE2=foobar2
+```
+
+Then, let's specify the environment variable without its value:
+
+```
+docker run --rm --env VARIABLE2 alpine env
+```
+
+And we can see Docker still picked up the value, this time from the surrounding environment:
+
+```
+VARIABLE2=foobar2
+```
+
+### Using --env-file
+
+The above solution is adequate when the number of variables is low. However, as soon as we have more than a handful of variables, it can quickly become cumbersome and error-prone.
+
+**An alternative solution is to use a text file to store our variables**, using the standard _key=value_ format.
+
+Let's define a few variables in a file we'll call _my-env.txt_:
+
+
+```
+$ echo VARIABLE1=foobar1 > my-env.txt
+$ echo VARIABLE2=foobar2 >> my-env.txt
+$ echo VARIABLE3=foobar3 >> my-env.txt
+```
+
+Now, let's inject this file into our Docker container:
+
+```
+$ docker run --env-file my-env.txt alpine:3 env
+```
+
+Finally, let's take a look at the output:
+
+```
+VARIABLE1=foobar1
+VARIABLE2=foobar2
+VARIABLE3=foobar3
+```

mkdocs.yml

@@ -39,6 +39,8 @@ nav:
     - Docker volumes and bind-mounts: 'container/volumes.md'
     - Docker volume plugins - an example: 'container/volume_plugin.md'
     - Summary: 'container/volume_summary.md'
+  - Environment variables:
+    - How to pass env variables to a container: 'container/env_vars.md'
   - Docker web UI:
     - Portainer: 'gui/portainer.md'