chore: added bootstrap scripts

app/bootstrap-app-bundle.sh

+#!/bin/sh
+set -ex
+bundle config --local build.sassc --disable-march-tune-native
+gem install bundler -v 2.1
+bundle install --jobs 20 --retry 5

app/bootstrap-app-dir.sh

+#!/bin/sh
+set -ex
+# Common bootstrap for a nexus application in a container.
+adduser nexus --system --group -u 1000
+chown -R nexus:nexus /app

app/bootstrap-apt-clean.sh

+#!/bin/sh
+set -ex
+apt-get autoremove
+apt-get autoclean
+apt-get clean
+rm -rf /var/lib/apt/lists/*

app/bootstrap-apt-upgrade.sh

+#!/bin/sh
+set -ex
+apt-get update
+DEBIAN_FRONTEND=noninteractive apt-get \
+  -o Dpkg::Options::="--force-confnew" --allow-remove-essential --allow-change-held-packages -fuy \
+  upgrade