Add missing environment variable for secure endpoint

[kinoute]

Following #512, we need to add support for the "secure endpoint" parameter through environment variables. I also added a default value for the chunked request size (found in the java code source) since an empty value will raise an exception:

[s3proxy] E 04-20 10:16:36.854 main org.gaul.s3proxy.Main:279 |::] java.lang.NumberFormatException: For input string: ""
[s3proxy] E 04-20 10:16:36.855 main org.gaul.s3proxy.Main:279 |::]     at java.base/java.lang.NumberFormatException.forInputSt
[s3proxy] E 04-20 10:16:36.855 main org.gaul.s3proxy.Main:279 |::]     at java.base/java.lang.Long.parseLong(Unknown Source)
[s3proxy] E 04-20 10:16:36.855 main org.gaul.s3proxy.Main:279 |::]     at java.base/java.lang.Long.parseLong(Unknown Source)
[s3proxy] E 04-20 10:16:36.856 main org.gaul.s3proxy.Main:279 |::]     at org.gaul.s3proxy.S3Proxy$Builder.fromProperties(S3Pr
[s3proxy] E 04-20 10:16:36.856 main org.gaul.s3proxy.Main:279 |::]     at org.gaul.s3proxy.Main.main(Main.java:158)

[kinoute]

Also added the 443 port to the Dockerfile

[gaul]

Could you rebase this PR to resolve the conflicts?

Also could you add some documentation to README.md on how users need to configure the keystore and their clients?

[kinoute]

I edited the SSL Support page and the README.md file to add a link to this page: https://github.com/gaul/s3proxy/wiki/SSL-support

[gaul]

Thank you for your contribution @kinoute!