build,.github/workflows: use go1.15

Change-Id: Ibcc70d56d493db997fad28e26bf9df8d6e01a6f3
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/248077
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
diff --git a/.github/workflows/release-nightly.yml b/.github/workflows/release-nightly.yml
index c5f6dcf..a05e999 100644
--- a/.github/workflows/release-nightly.yml
+++ b/.github/workflows/release-nightly.yml
@@ -29,7 +29,7 @@
       - name: Setup Go
         uses: actions/setup-go@v1
         with:
-         go-version: '1.14'
+         go-version: '1.15'
 
       - name: Install dependencies
         run: npm ci
diff --git a/.github/workflows/test-long.yml b/.github/workflows/test-long.yml
index 06ed97f..bd4d631 100644
--- a/.github/workflows/test-long.yml
+++ b/.github/workflows/test-long.yml
@@ -33,7 +33,7 @@
       - name: Setup Go
         uses: actions/setup-go@v1
         with:
-         go-version: '1.14'
+         go-version: '1.15'
 
       - name: Install dependencies
         run: npm ci
diff --git a/.github/workflows/test-smoke.yml b/.github/workflows/test-smoke.yml
index 4a74de6..6a1d2c7 100644
--- a/.github/workflows/test-smoke.yml
+++ b/.github/workflows/test-smoke.yml
@@ -29,7 +29,7 @@
       - name: Setup Go
         uses: actions/setup-go@v1
         with:
-         go-version: '1.14'
+         go-version: '1.15'
 
       - name: Install dependencies
         run: npm ci
@@ -68,4 +68,4 @@
 
       - name: Lint check
         run: npm run lint
-        if: ${{ matrix.os }} == 'ubuntu-latest'
\ No newline at end of file
+        if: ${{ matrix.os }} == 'ubuntu-latest'
diff --git a/build/Dockerfile b/build/Dockerfile
index e2b3a71..b9cc9f6 100644
--- a/build/Dockerfile
+++ b/build/Dockerfile
@@ -1,6 +1,6 @@
 # This Docker container is used for testing on GCB.
 
-FROM golang:1.14 AS gobuilder
+FROM golang:1.15 AS gobuilder
 
 ENV GO111MODULE on
 ENV GOBIN /gobin
@@ -14,7 +14,8 @@
 COPY --from=gobuilder /gobin /go/bin
 COPY --from=gobuilder /usr/local/go /usr/local/go
 
-ENV PATH=${GOPATH}/bin:/usr/local/go/bin:${PATH}
+ENV GOPATH /go
+ENV PATH ${GOPATH}/bin:/usr/local/go/bin:${PATH}
 ENV DEBIAN_FRONTEND noninteractive
 
 RUN apt-get update && apt-get install -y libnss3 libgtk-3-dev libxss1 libasound2 xvfb libsecret-1-0