Merge pull request #95 from indigo-dc/devel

Devel

doc/reference_card.md

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+udocker reference card
+======================
+udocker is user oriented tool to enable pulling and execution of docker
+containers were docker is unavailable or cannot be used safely. 
+
+## Configuration files
+
+* /etc/udocker.conf
+* $UDOCKER_DIR/udocker.conf
+* $HOME/.udocker/udocker.conf
+
+All attributes of the udocker::Config class can be modified via the
+configuration files. Example:
+
+```
+# do not verify digital certificates
+http_insecure = True
+# set default verbose level
+verbose_level = 5
+```
+
+## Environment variables
+
+ * UDOCKER_DIR : root directory of udocker usually $HOME/.udocker
+ * UDOCKER_BIN : location of udocker related executables
+ * UDOCKER_LIB : location of udocker related libraries
+ * UDOCKER_CONTAINERS : location of container directory trees (not images)
+ * UDOCKER_KEYSTORE : location of keystore for repository login/logout
+ * UDOCKER_TMP : location of temporary directory
+ * UDOCKER_TARBALL : location of installation tarball (file of URL)
+ * UDOCKER_LOGLEVEL : logging level
+
+## Verbosity
+
+The verbosity level of udocker can be enforced. Removing banners and most
+messages can be achieved by executing with UDOCKER_LOGLEVEL=2
+
+ * UDOCKER_LOGLEVEL : set verbosity level from 0 to 5 (MIN to MAX verbosity)
+
+Optionally invoke udocker with `--quiet` or `-q` to decrease verbosity.
+
+```
+ udocker --quiet run <container>
+```
+
+## Security
+
+udocker does not require any type of privileges nor the deployment of 
+services by system administrators. It can be downloaded and executed 
+entirely by the end user. udocker runs under the identity of the user
+invoking it.
+
+Most udocker execution modes do not provide process isolation features
+such as docker. Due to the lack of isolation udocker must not be run 
+by privileged users.
+
+## Troubleshooting
+
+Invoke udocker with `-D` for debugging.
+
+```
+ udocker -D run <container>
+```
+
+## Documentation
+
+* https://www.gitbook.com/book/indigo-dc/udocker/details
+* https://github.com/indigo-dc/udocker/blob/master/SUMMARY.md 
+