gometalinter: add per-platform configurable options

I have run into two separate issues while doing 'make all' on armhf
(a Scaleway C1 machine, same as used in CI). This commit fixes both.

1. There were a lot of "not enough memory" errors, and after that
in a few runs gometalinter just stuck forever on FUTEX_WAIT with
no children left.

Looking into docs, I found the --enable-gc option which solved the issue.

[Update: this has already been added]

2. Timeout of 2 minutes is not enough for the abovementioned platform.
The longest running linter is goimports which takes almost 6 minutes to run.

Set the timeout to the observable run time roughly doubled.

In addition, ARM platforms does not have too much RAM (2GB), so
running too many processes in parallel might be problematic. Limit
it by using -j2

[v2: make the timeout arch-dependent, also tested on aarch64 (2m15s)]
[v3: moved timeout setting to Dockerfiles]
[v4: generalized to GOMETALINTER_OPTS, added -j2 for ARM platforms]
[v5: rebase to master]

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

Dockerfile

@@ -216,3 +216,6 @@ ENTRYPOINT ["hack/dind"]
 
 # Upload docker source
 COPY . /go/src/github.com/docker/docker
+
+# Options for hack/validate/gometalinter
+ENV GOMETALINTER_OPTS="--deadline 2m"

Dockerfile.aarch64

@@ -181,3 +181,6 @@ ENTRYPOINT ["hack/dind"]
 
 # Upload docker source
 COPY . /go/src/github.com/docker/docker
+
+# Options for hack/validate/gometalinter
+ENV GOMETALINTER_OPTS="--deadline 4m -j2"

Dockerfile.armhf

@@ -169,3 +169,6 @@ ENTRYPOINT ["hack/dind"]
 
 # Upload docker source
 COPY . /go/src/github.com/docker/docker
+
+# Options for hack/validate/gometalinter
+ENV GOMETALINTER_OPTS="--deadline 10m -j2"

hack/validate/gometalinter

@@ -3,4 +3,9 @@ set -e -o pipefail
 
 SCRIPTDIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
 
-gometalinter --config $SCRIPTDIR/gometalinter.json ./...
+# CI platforms differ, so per-platform GOMETALINTER_OPTS can be set
+# from a platform-specific Dockerfile, otherwise let's just set
+# (somewhat pessimistic) default of 10 minutes.
+gometalinter \
+	${GOMETALINTER_OPTS:--deadine 10m} \
+	--config $SCRIPTDIR/gometalinter.json ./...

hack/validate/gometalinter.json

@@ -1,6 +1,5 @@
 {
   "Vendor": true,
-  "Deadline": "2m",
   "EnableGC": true,
   "Sort": ["linter", "severity", "path"],
   "Exclude": [