Update docker.md for SMTP support

[sventhiel]

I tried to setup Exceptionless with SMTP support on my own machine. Unfortunately, to use ONLY the following environmental parameter does not work (see exceptionless/Exceptionless#1475, more precisely the comment from @gianlucamasini - exceptionless/Exceptionless#1475 (comment)

-e EX_ConnectionStrings__Email=smtps://user:password@smtp.host.com:587 \

So I had to add the mentioned parameters as well to get the smtp up and running.

-e EX_ConnectionStrings__Email=[smtp | smtps]://user:password@smtp.host.com:587 \
-e EX_AppMode="[Development | Staging | Production]" \
-e EX_BaseURL="[http | https]://example.com" \
-e EX_SmtpFrom="John Doe <john.doe@example.com>" \

For convinience and ease, it might be a good try to add those parameters to your docker guideline as well.

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