Dockerfile.build: improve docker compatibility

Try to fix:
```
Step 6/18 : COPY     ceph.spec.in     do_cmake.sh     install-deps.sh     run-make-check.sh     src/script/buildcontainer-setup.sh     ${CEPH_CTR_SRC}
When using COPY with more than one source file, the destination must be a directory and end with a /
```

Signed-off-by: John Mulligan <[email protected]>

Author: phlogistonjohn

Dockerfile.build

@@ -13,7 +13,7 @@ COPY \
     install-deps.sh \
     run-make-check.sh \
     src/script/buildcontainer-setup.sh \
-    ${CEPH_CTR_SRC}
+    ${CEPH_CTR_SRC}/
 
 
 FROM $DISTRO