Could not create parent directory for lock file when a non-root user

**How to run the image using a non-root user offline?**

The previously downloaded already locally available gradle distribution (downloaded at the moment of building the https://github.com/docToolchain/docker-image/blob/master/alpine/Dockerfile) should be used.

# Problem
I integrated this docker-image in my Jenkins-CI pipeline like this.

```
pipeline {

agent {
docker {
image 'rdmueller/doctoolchain:rc-1.2.0'
}
}

stages {
stage ("Generate PDFs") {
steps {
sh "doctoolchain . generatePDF"
}
}
}
```
Which is equivalent to
```
#!/usr/bin/env bash
docker run --user jenkinbuilduser --rm -it --entrypoint /bin/bash -v ${PWD}:/project rdmueller/doctoolchain:rc-1.2.0 \
-c "doctoolchain . $1 $2 $3 $4 $5 $6 $7 $8 $9 -PinputPath=src/main/asciidoc -PmainConfigFile=config/docToolchain.groovy && exit"
```
or
```
#!/usr/bin/env bash
docker run --user 501 --rm -it --entrypoint /bin/bash -v ${PWD}:/project rdmueller/doctoolchain:rc-1.2.0 \
-c "doctoolchain . $1 $2 $3 $4 $5 $6 $7 $8 $9 -PinputPath=src/main/asciidoc -PmainConfigFile=config/docToolchain.groovy && exit"
```

from your documentation. Note the added option `--user jenkinbuilduser` and `--user 501`.

But this leads to exceptions like
```
Exception in thread "main" java.lang.RuntimeException: Could not create parent directory for lock file /docToolchain/?/.gradle/wrapper/dists/gradle-6.0.1-bin/1lxlpkiy24sb18odw96cp4ojv/gradle-6.0.1-bin.zip.lck
at org.gradle.wrapper.ExclusiveFileAccessManager.access(ExclusiveFileAccessManager.java:43)
at org.gradle.wrapper.Install.createDist(Install.java:48)
at org.gradle.wrapper.WrapperExecutor.execute(WrapperExecutor.java:107)
at org.gradle.wrapper.GradleWrapperMain.main(GradleWrapperMain.java:63)
```

My investigations showed that `/root/.gradle` (which holds the downloaded gradle distribution created by https://github.com/docToolchain/docker-image/blob/master/alpine/Dockerfile#L41) is not accessible by a non-root user like `jenkinbuilduser`. gradlew interprets it that no distribution is locally available and starts the download of the gradle distribution into `/docToolchain/?/.gradle`. This directory is read-only, which leads to the exception.

# First workaround (non-working)
My first workaround was to run the image as user root.
```
pipeline {

agent {
docker {
image 'rdmueller/doctoolchain:rc-1.2.0'
args """-u root """
}
}

stages {
stage ("Generate PDFs") {
steps {
sh "doctoolchain . generatePDF"
}
}
}
```
Which is equivalent to
```
#!/usr/bin/env bash
docker run --rm -it --user root --entrypoint /bin/bash -v ${PWD}:/project rdmueller/doctoolchain:rc-1.2.0 \
-c "doctoolchain . $1 $2 $3 $4 $5 $6 $7 $8 $9 -PinputPath=src/main/asciidoc -PmainConfigFile=config/docToolchain.groovy && exit"
```
(Note the option `--user root`)

This way there is no exception, the generation works, but the generated file are owned by user root on the host file-system. This is not acceptable, because these files cannot be removed anymore (at least not by a next build steps within the build job. It can only be deleted manually by a fellow administrator with root-user rights)

# Second workaround (partially working)

Set a non-root user and a rw-directory for the gradle distribution.
```
#!/usr/bin/env bash
docker run --user 501 -e GRADLE_USER_HOME='/project/.gradlecustom' --rm -it ---entrypoint /bin/bash -v ${PWD}:/project rdmueller/doctoolchain:rc-1.2.0 \
-c "doctoolchain . $1 $2 $3 $4 $5 $6 $7 $8 $9 -PinputPath=src/main/asciidoc -PmainConfigFile=config/docToolchain.groovy && exit"
```
gradlew will download a distribution to `/project/.gradlecustom` , the generation succeeds and the generated files are owned by a non-root user.

*The problem*: A buildjob should not have access to the internet.

# The remaining problem
The issue is still remaining: How to run the image using a non-root user offline. The previously downloaded already locally available gradle distribution should be used.