lxdock shell -c does not use login environment and sets incorrect SHELL

[shuhaowu]

When executing a command via lxdock shell -c, it is not providing an environment similar to as if user logged in directly. This is because we are using su -l <user> -s, then passing a file that has #!/bin/sh. This overrides the login environment as we're now in the hands of /bin/sh.

To fix this, we can probably use #!/bin/sh -l, although we might encounter compatibility issues if people's profile are only compatible with bash and equivalent and sh is something like dash.

Also, $SHELL is set to the script that we create, which is problematic for programs that detect $SHELL

[robvdl]

We set SHELL now, not sure if anything else needs to be done. Surely there must be more login environment variables we're still missing.

[mtausig]

@shuhaowu Do you have an example script which previously failed, to validate if #173 was enough to close this issue?