Cannot apt-get in docker executors

I have followed the guides to setup gitlab-ee:

https://docs.gitlab.com/runner/install/linux-repository.html#upgrading-to-gitlab-runner-10
https://docs.gitlab.com/runner/register/index.html#gnu-linux

I can run ci, but whenever I try to install new packages (using apt-get on debian-based docker containers, or apk on alpine-based docker containers), I do get permission denied. It seems I don’t have root permissions in my docker containers. How can I setup the docker containers so I can install packages?

(using the docker image gradle/alpine, I can use gradle to download java packages, but no luck with downloading debs:

$ gradle --build-cache assemble
Download https://plugins.gradle.org/m2/ru/vyarus/quality/ru.vyarus.quality.gradle.plugin/2.4.0/ru.vyarus.quality.gradle.plugin-2.4.0.pom
Download https://plugins.gradle.org/m2/ru/vyarus/gradle-quality-plugin/2.4.0/gradle-quality-plugin-2.4.0.pom
Download https://plugins.gradle.org/m2/ru/vyarus/gradle-quality-plugin/2.4.0/gradle-quality-plugin-2.4.0.jar
:compileJava
Download https://jcenter.bintray.com/org/slf4j/slf4j-api/1.7.25/slf4j-api-1.7.25.pom
Download https://jcenter.bintray.com/org/slf4j/slf4j-parent/1.7.25/slf4j-parent-1.7.25.pom
...

)

Thx.

$ sudo cat /etc/gitlab-runner/config.toml
concurrent = 5
check_interval = 0

[[runners]]
name = "..."
url = "..."
token = "..."
executor = "docker"
[runners.docker]
tls_verify = false
image = "alpine:latest"
privileged = true
disable_cache = false
volumes = ["/cache"]
shm_size = 0
[runners.cache]
$ apt-get update -qy && apt-get upgrade
Reading package lists...
E: List directory /var/lib/apt/lists/partial is missing. - Acquire (13: Permission denied)