Use a different variable for svn than wget

The master branch is called trunk in svn parlance
and the directory structure is a little bit different.

Also trust the certificates for the https svn connections,
otherwise we may be prompted.

Dockerfile

@@ -15,10 +15,13 @@ COPY . md_source
 # then nuke the md_source directory.
 
 ## Branch to pull from, per ref doc
+## To get master from svn the svn branch needs to be 'trunk'. To get a branch from svn it needs to be 'branches/branchname'
+ENV ENGINE_SVN_BRANCH="branches/17.04.x"
 ENV ENGINE_BRANCH="17.04.x"
+ENV DISTRIBUTION_SVN_BRANCH="branches/release/2.6"
 ENV DISTRIBUTION_BRANCH="release/2.6"
 
-RUN svn co https://github.com/docker/docker/branches/$ENGINE_BRANCH/docs/extend md_source/engine/extend \
+RUN svn co https://github.com/docker/docker/branches/$ENGINE_SVN_BRANCH/docs/extend md_source/engine/extend \
 	&& wget -O md_source/engine/api/v1.18.md https://raw.githubusercontent.com/docker/docker/$ENGINE_BRANCH/docs/api/v1.18.md \
 	&& wget -O md_source/engine/api/v1.19.md https://raw.githubusercontent.com/docker/docker/$ENGINE_BRANCH/docs/api/v1.19.md \
 	&& wget -O md_source/engine/api/v1.20.md https://raw.githubusercontent.com/docker/docker/$ENGINE_BRANCH/docs/api/v1.20.md \
@@ -32,7 +35,7 @@ RUN svn co https://github.com/docker/docker/branches/$ENGINE_BRANCH/docs/extend
 	&& wget -O md_source/engine/reference/run.md https://raw.githubusercontent.com/docker/docker/$ENGINE_BRANCH/docs/reference/run.md \
 	&& wget -O md_source/engine/reference/commandline/cli.md https://raw.githubusercontent.com/docker/docker/$ENGINE_BRANCH/docs/reference/commandline/cli.md \
 	&& wget -O md_source/engine/deprecated.md https://raw.githubusercontent.com/docker/docker/$ENGINE_BRANCH/docs/deprecated.md \
-	&& svn co https://github.com/docker/distribution/branches/$DISTRIBUTION_BRANCH/docs/spec md_source/registry/spec \
+	&& svn co https://github.com/docker/distribution/branches/$DISTRIBUTION_SVN_BRANCH/docs/spec md_source/registry/spec \
 	&& rm md_source/registry/spec/api.md.tmpl \
 	&& wget -O md_source/registry/configuration.md https://raw.githubusercontent.com/docker/distribution/$DISTRIBUTION_BRANCH/docs/configuration.md \
 	&& rm -rf md_source/apidocs/cloud-api-source \