Docker: Make install-plugins.sh compatible with plugins.txt input file in plugins.sh

[Trane9991]

Hello,
make possible to pass plugins.txt file in the old format to the install-plugins.sh script:

plugins.txt:

...
jacoco:1.0.19
gitlab
...

and run with:
./install-plugins.sh plugins.txt

[duckpuppy]

As a workaround until the enhancement is in, I'll toss this one-liner here:
RUN /usr/local/bin/install-plugins.sh $(cat /usr/share/jenkins/plugins.txt | tr '\n' ' ')