fix(docker): requiring docker >20 for --build-context support (#207)

* fix(docker): requiring docker >20 for --build-context support

looks like even if buildx is installed in older docker versions,
--build-context arg is not supported

cant find release docs for this doc so just going by local version
experiments

* fix: wrong space in COPY --chmod docker statement

CHANGELOG.md

@@ -19,6 +19,9 @@
   As a result other commands such as `exec` do not interact
   with the backup service.
   [#191](https://github.com/crashappsec/chalk/pull/191)
+- Fixing docker build attempting to use `--build-context`
+  on older docker versions which did not support that flag.
+  [#207](https://github.com/crashappsec/chalk/pull/207)
 
 ## 0.3.2
 

src/docker_base.nim

@@ -101,7 +101,7 @@ template hasBuildx*(): bool =
 
 template supportsBuildContextFlag*(): bool =
   # https://github.com/docker/buildx/releases/tag/v0.8.0
-  getBuildXVersion() >= parseVersion("0.8")
+  getDockerVersion() >= parseVersion("21") and getBuildXVersion() >= parseVersion("0.8")
 
 template supportsCopyChmod*(): bool =
   # > the --chmod option requires BuildKit.
@@ -149,6 +149,8 @@ proc makeFileAvailableToDocker*(ctx:     DockerInvocation,
   if hasUser and chmod == "":
     chmod = "0444"
 
+  let chmodstr = if chmod == "": "" else: "--chmod=" & chmod & " "
+
   if move:
     trace("Making file available to docker via move: " & loc)
   else:
@@ -158,10 +160,6 @@ proc makeFileAvailableToDocker*(ctx:     DockerInvocation,
     once:
       trace("Docker injection method: --build-context")
 
-    var chmodstr = ""
-    if chmod != "":
-      chmodstr = "--chmod=" & chmod & " "
-
     ctx.newCmdLine.add("--build-context")
     ctx.newCmdLine.add("chalkexedir" & $(contextCounter) & "=" & dir & "")
     ctx.addedInstructions.add("COPY " & chmodstr & "--from=chalkexedir" &
@@ -198,8 +196,8 @@ proc makeFileAvailableToDocker*(ctx:     DockerInvocation,
         copyFile(loc, dstLoc)
         trace("Copied " & loc & " to " & dstLoc)
 
-      if chmod != "" and supportsCopyChmod():
-        ctx.addedInstructions.add("COPY --chmod= " & chmod &
+      if chmodstr != "" and supportsCopyChmod():
+        ctx.addedInstructions.add("COPY " & chmodstr &
                                   file & " " & " /" & newname)
       elif chmod != "":
         # TODO detect user from base image if possible but thats not