Fix --no-container, add example combining Conda + explicit docker options.

Sorry about that @jxtx.

This:

```
cwltool --no-container --beta-conda-dependencies tests/seqtk_seq_with_docker.cwl tests/seqtk_seq_job.json
```

is working for me now.

Author: jmchilton

cwltool/draft2tool.py

@@ -286,7 +286,7 @@ def rm_pending_output_callback(output_callbacks, jobcachepending,
 
         reffiles = copy.deepcopy(builder.files)
 
-        j = self.makeJobRunner(kwargs.get("find_default_container"))
+        j = self.makeJobRunner(kwargs.get("use_container"))
         j.builder = builder
         j.joborder = builder.job
         j.stdin = None

tests/seqtk_seq_with_docker.cwl

@@ -0,0 +1,26 @@
+cwlVersion: v1.0
+class: CommandLineTool
+id: "seqtk_seq"
+doc: "Convert to FASTA (seqtk)"
+inputs:
+  - id: input1
+    type: File
+    inputBinding:
+      position: 1
+      prefix: "-a"
+outputs:
+  - id: output1
+    type: File
+    outputBinding:
+      glob: out
+baseCommand: ["seqtk", "seq"]
+arguments: []
+stdout: out
+hints:
+  SoftwareRequirement:
+    packages:
+    - package: seqtk
+      version:
+      - '1.2'
+  DockerRequirement:
+    dockerPull: quay.io/biocontainers/seqtk:1.2--0