diff --git a/Dockerfile b/Dockerfile
index 1b0c577..2d55e1c 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -85,16 +85,21 @@ RUN set -x \
  # override environment to run redmine with a context path "/redmine"
  && mv ${WORKDIR}/config/environment.ces.rb ${WORKDIR}/config/environment.rb \
 
- # install core plugins to a temporary location
- # besure plugin name does not contain a minus or dots,
- # because the minus separates the version from the package name
- && mkdir -p /var/tmp/redmine/plugins \
+ # install core plugins
+ && mkdir -p "${WORKDIR}/plugins" \
+ # install cas plugin
+ && mkdir "${WORKDIR}/plugins/redmine_cas" \
  && curl -sL \
     https://github.com/cloudogu/redmine_cas/archive/${CAS_PLUGIN_VERSION}.tar.gz \
-    -o /var/tmp/redmine/plugins/redmine_cas-${CAS_PLUGIN_VERSION}.tar.gz \
+  | tar xfz - --strip-components=1 -C "${WORKDIR}/plugins/redmine_cas" \
+ # install redmine_activerecord_session_store to be able to invalidate sessions after cas logout
+ && mkdir "${WORKDIR}/plugins/redmine_activerecord_session_store" \
  && curl -sL \
     https://github.com/pencil/redmine_activerecord_session_store/archive/v${ACTIVERECORD_SESSION_STORE_PLUGIN_VERSION}.tar.gz \
-    -o /var/tmp/redmine/plugins/redmine_activerecord_session_store-${ACTIVERECORD_SESSION_STORE_PLUGIN_VERSION}.tar.gz \
+  | tar xfz - --strip-components=1 -C "${WORKDIR}/plugins/redmine_activerecord_session_store" \
+
+ # install plugin gems
+ && cd ${WORKDIR}; RAILS_ENV="production" bundle install --without development test \
 
  # cleanup
  && rm -rf /root/* /tmp/* $(gem env gemdir)/cache \
diff --git a/dogu.json b/dogu.json
index 302f456..fc907ed 100644
--- a/dogu.json
+++ b/dogu.json
@@ -20,7 +20,7 @@
     "Group": "1000"
   }, {
     "Name": "plugins",
-    "Path": "/usr/share/webapps/redmine/plugins",
+    "Path": "/var/tmp/redmine/plugins",
     "Owner": "1000",
     "Group": "1000"
   }, {
@@ -37,6 +37,9 @@
     "Port": 3000
   },
   "ExposedCommands": [{
+    "Name": "pre-upgrade",
+    "Command": "/pre-upgrade.sh"
+  },{
     "Name": "post-upgrade",
     "Command": "/post-upgrade.sh"
   }]
diff --git a/resources/post-upgrade.sh b/resources/post-upgrade.sh
index 379619b..ab0cb72 100755
--- a/resources/post-upgrade.sh
+++ b/resources/post-upgrade.sh
@@ -16,9 +16,6 @@ function exec_rake() {
 
 echo "executing update ${FROM_VERSION} to ${TO_VERSION}"
 
-echo "install dependencies ..."
-RAILS_ENV="production" REDMINE_LANG="en" bundle install
-
 echo "migrate database ..."
 exec_rake db:migrate
 
diff --git a/resources/pre-upgrade.sh b/resources/pre-upgrade.sh
new file mode 100755
index 0000000..a646c9b
--- /dev/null
+++ b/resources/pre-upgrade.sh
@@ -0,0 +1,20 @@
+#!/bin/bash
+
+# older installation of redmine exposed the core plugins to the
+# plugin volume, these exposed plugin could be old and must be
+# removed.
+
+set -o errexit
+set -o nounset
+set -o pipefail
+
+PLUGIN_DIRECTORY="${WORKDIR}/plugins"
+CORE_PLUGINS="redmine_activerecord_session_store redmine_cas"
+
+for CP in ${CORE_PLUGINS}; do
+  CP_DIR="${PLUGIN_DIRECTORY}/${CP}"
+  if [ -d "${CP_DIR}" ]; then
+    echo "remove old installation of ${CP}"
+    rm -rf "${CP_DIR}"
+  fi
+done
\ No newline at end of file
diff --git a/resources/startup.sh b/resources/startup.sh
index 7c520f6..eb0ccbc 100755
--- a/resources/startup.sh
+++ b/resources/startup.sh
@@ -36,14 +36,20 @@ function exec_rake() {
 
 function install_plugins(){
   echo "installing plugins"
-  for PLUGIN_PACKAGE in $(ls "${PLUGIN_STORE}"); do
+
+  PLUGINS=$(ls "${PLUGIN_STORE}")
+  for PLUGIN_PACKAGE in ${PLUGINS}; do
     install_plugin "${PLUGIN_PACKAGE}"
   done
 
-  echo "install missing gems ..."
-  RAILS_ENV="${RAILS_ENV}" REDMINE_LANG="${REDMINE_LANG}" bundle install
-  echo "missing gems ... installed"
+  # install missing gems only if external plugins are going to install
+  if [ "x${PLUGINS}" != "x" ]; then
+    echo "install missing gems ..."
+    RAILS_ENV="${RAILS_ENV}" REDMINE_LANG="${REDMINE_LANG}" bundle install
+    echo "missing gems ... installed"
+  fi
 
+  # run migrations always, because core plugins need also a db migration
   echo "running plugin migrations..."
   exec_rake redmine:plugins:migrate
   echo "plugin migrations... done"
@@ -51,18 +57,20 @@ function install_plugins(){
 
 # installs or upgrades the given plugin
 function install_plugin(){
-  PACKAGE="${1}"
-  NAME=$(echo "${PACKAGE}" | awk -F'-' '{print $1}')
-  VERSION=$(echo "${PACKAGE}" | awk -F'-' '{print $NF}' | sed -e 's/\.tar\.gz//g')
-  DIRECTORY="${PLUGIN_DIRECTORY}/${NAME}"
-
-  echo "install plugin ${NAME} in version ${VERSION}"
-  if [ -d "${DIRECTORY}" ]; then
-    rm -rf "${DIRECTORY}"  
+  NAME="${1}"
+  SOURCE="${PLUGIN_STORE}/${NAME}"
+  TARGET="${PLUGIN_DIRECTORY}/${NAME}"
+
+  if [ ! -d "${SOURCE}" ]; then
+    exit 1
   fi
 
-  mkdir -p "${DIRECTORY}"
-  tar xfz "${PLUGIN_STORE}/${PACKAGE}" --strip-components=1 -C "${DIRECTORY}" > /dev/null 2>&1
+  echo "install plugin ${NAME}"
+  if [ -d "${TARGET}" ]; then
+    rm -rf "${TARGET}"  
+  fi
+
+  cp -rf "${SOURCE}" "${TARGET}"
 }
 
 # adjust redmine database.yml
@@ -141,8 +149,7 @@ else
   sql "DELETE FROM users WHERE login='admin';"
 fi
 
-
-# install core plugins and manual installed plugins
+# install manual installed plugins
 install_plugins
 
 # Create links