install-docker.sh: use registry mirror mirror.gcr.io

See merge request !1

`... icinga-build-test testing/live_test.sh`

For older versions of bash...

So that older distributions can be used

This reverts commit 809f9be3.

go get can create read-only files

This reverts commit 02c6a2ca.

If requires are empty for source.

To avoid issues with mirrors of distributions

Also clean an empty ccache dir.

Replaced by install-docker.sh