Relax disk space checking to not abort in cases where df output sho…

…ws no available volumes (see #273). While the cause is not known, and Docker may yet fail, relaxing this constraint at least lets Docker try without aborting prematurely.

core/src/main/java/org/testcontainers/DockerClientFactory.java

@@ -157,7 +157,7 @@ private void checkDiskSpace(DockerClient client) {
                     df.usedPercent.map(x -> x.toString() + "%").orElse("unknown"), 
                     df.availableMB.map(x -> x.toString() + " MB available").orElse("unknown available"));
 
-            if (df.availableMB.orElseThrow(NotAbleToGetDiskSpaceUsageException::new) < 2048) {
+            if (df.availableMB.isPresent() && df.availableMB.get() < 2048) {
                 LOGGER.error("Docker environment has less than 2GB free - execution is unlikely to succeed so will be aborted.");
                 throw new NotEnoughDiskSpaceException("Not enough disk space in Docker environment");
             }
@@ -224,12 +224,6 @@ private static class NotEnoughDiskSpaceException extends RuntimeException {
             super(message);
         }
     }
-
-    private static class NotAbleToGetDiskSpaceUsageException extends RuntimeException {
-        NotAbleToGetDiskSpaceUsageException() {
-            super();
-        }
-    }
 }
 
 class LogContainerCallback extends LogContainerResultCallback {